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/assets/stylesheets/components/table.scss

@@ -73,6 +73,10 @@
 
   }
 
+  &-index {
+    width: 15px;
+  }
+
 }
 
 .table-field-heading {