Site Editor: Use <table> in templates list page

[noisysocks]

The templates list page added in #36379 uses a <ul> for the markup that displays the templates available to edit.

<table> is the more semantically correct element to use here as we are presenting tabular data ("data with multiple dimensions") to the user. It also assists screen readers, as @talldan summarises in #36379 (comment):

When tabbing through a table, a screen reader user will be informed which column and row they're in, and when entering the table for the first time how many columns and rows there are. With a list, the user will only know how many list items, and won't be aware of columns.