Add row numbers to tables

When a table is showing the contents of a CSV file, it should look
something like a spreadsheet.

The minimally skeuomorphic way to do this is by adding row numbers.

This commit doesn’t
- make the row numbers monospace (it’s barely noticeable and doesn’t
  reflect what actual spreadsheets do)
- make the first column heading ‘Row’ (again, doesn’t reflect how actual
  spreadsheets work, and takes up more valuable space)

app/templates/components/table.html

@@ -29,7 +29,7 @@
   {% call mapping_table(caption, field_headings, field_headings_visible, caption_visible) %}
     {% for item in items %}
       {% call row() %}
-        {{ parent_caller(item) }}
+        {{ parent_caller(item, loop.index + 1) }}
       {% endcall %}
     {% endfor %}
     {% if not items %}
@@ -55,6 +55,12 @@
   </td>
 {%- endmacro %}
 
+{% macro index_field(text) -%}
+  <td class="table-field-index">
+    <span>{{ text }}</span>
+  </td>
+{%- endmacro %}
+
 {% macro text_field(text) -%}
   {% call field() %}
     {{ text }}