Change tables to scroll in-page, not full screen

There were three problems with showing tables fullscreen:
- it was over-optimised for very big spreadsheets, whereas most users
  will only have a few columns in their files
- it was jarring to go from full screen and back to the normal layout
- it was a bit change for existing users, where we prefer incremental
  changes that make things better without disrupting people’s work
  (where possible)

So this commit changes the big table to scroll horizontally in the page,
not take up the full width of the page.

From the fullscreen table it keeps:
- the shimming method to keep the horizontal scrollbar at the bottom of
  the screen at all times

It introduces some more refinements to make it nicer to use:
- fixing the first column, so you always know what row you’re on
- adding shadows indicate where there is content that’s scrolled outside
  the edges of the container

app/templates/views/check/column-errors.html

@@ -114,54 +114,58 @@
     {% endcall %}
   </div>
 
-  <div class="bottom-gutter-3-2">
-    {% if request.args.from_test %}
-      <a href="{{ back_link }}" class="page-footer-back-link">Back</a>
-    {% else %}
-      {{file_upload(form.file, button_text='Re-upload your file')}}
-    {% endif %}
+
+  <div class="js-stick-at-top-when-scrolling">
+    <div class="form-group">
+      {% if request.args.from_test %}
+        <a href="{{ back_link }}" class="page-footer-back-link">Back</a>
+      {% else %}
+        {{file_upload(form.file, button_text='Re-upload your file')}}
+      {% endif %}
+    </div>
+    <a href="#content" class="back-to-top-link">Back to top</a>
   </div>
 
   {% if not request.args.from_test %}
 
     <h2 class="heading-medium" id="{{ file_contents_header_id }}">{{ original_file_name }}</h2>
 
-    {% call(item, row_number) list_table(
-      recipients.initial_annotated_rows_with_errors if row_errors and not recipients.missing_column_headers else recipients.initial_annotated_rows,
-      caption=original_file_name,
-      caption_visible=False,
-      field_headings=[
-        '<span class="visually-hidden">Row in file</span><span aria-hidden="true">1</span>'|safe
-      ] + recipients.column_headers
-    ) %}
-      {% call index_field() %}
-        <span class="{% if item.index in recipients.rows_with_errors %}table-field-error{% endif %}">
-          {{ item.index + 2 }}
-        </span>
-      {% endcall %}
-      {% for column in recipients.column_headers %}
-        {% if item['columns'][column].error and not recipients.missing_column_headers %}
-          {% call field() %}
-            <span>
-              <span class="table-field-error-label">{{ item['columns'][column].error }}</span>
-              {{ item['columns'][column].data if item['columns'][column].data != None }}
-            </span>
-          {% endcall %}
-        {% elif item['columns'][column].ignore %}
-          {{ text_field(item['columns'][column].data or '', status='default') }}
-        {% else %}
-          {{ text_field(item['columns'][column].data or '') }}
-        {% endif %}
-      {% endfor %}
-      {% if item['columns'].get(None) %}
-        {% for column in item['columns'][None].data %}
-          {{ text_field(column, status='default') }}
+    <div class="fullscreen-content" data-module="fullscreen-table">
+      {% call(item, row_number) list_table(
+        recipients.initial_annotated_rows_with_errors if row_errors and not recipients.missing_column_headers else recipients.initial_annotated_rows,
+        caption=original_file_name,
+        caption_visible=False,
+        field_headings=[
+          '<span class="visually-hidden">Row in file</span><span aria-hidden="true">1</span>'|safe
+        ] + recipients.column_headers
+      ) %}
+        {% call index_field() %}
+          <span>
+            {{ item.index + 2 }}
+          </span>
+        {% endcall %}
+        {% for column in recipients.column_headers %}
+          {% if item['columns'][column].error and not recipients.missing_column_headers %}
+            {% call field() %}
+              <span>
+                <span class="table-field-error-label">{{ item['columns'][column].error }}</span>
+                {{ item['columns'][column].data if item['columns'][column].data != None }}
+              </span>
+            {% endcall %}
+          {% elif item['columns'][column].ignore %}
+            {{ text_field(item['columns'][column].data or '', status='default') }}
+          {% else %}
+            {{ text_field(item['columns'][column].data or '') }}
+          {% endif %}
         {% endfor %}
-      {% endif %}
-    {% endcall %}
-
-  {% endif %}
-
+        {% if item['columns'].get(None) %}
+          {% for column in item['columns'][None].data %}
+            {{ text_field(column, status='default') }}
+          {% endfor %}
+        {% endif %}
+      {% endcall %}
+    {% endif %}
+  </div>
 
   {% if recipients.too_many_rows %}
     <p class="table-show-more-link">