Use small font sizes in tables by default

GOV.UK Elements changed tables to be a larger font size here:
https://github.com/alphagov/govuk_elements/pull/185

This is good in principle (and a lot of our tables are 19px already).
However, the ones that aren’t are still 16px because there’s a lot of
info to fit on the page (eg when previewing someone’s CSV file).

app/templates/components/table.html

@@ -1,7 +1,7 @@
 {% from "components/big-number.html" import big_number %}
 
 {% macro mapping_table(caption='', field_headings=[], field_headings_visible=True, caption_visible=True) -%}
-  <table class="table">
+  <table class="table table-font-xsmall">
     <caption class="heading-medium table-heading{{ ' visuallyhidden' if not caption_visible}}">
       {{ caption }}
     </caption>