Theme test

Temporary fork of /extensions/themetest

This provides a page with copies of HTML snippets in use throughout CiviCRM. e.g. (at least pre 5.31) buttons were coded in many different ways; this extension has a copy of each type of button on one page so you can check if your theme renders them nicely. This was originally part of Artful Robot's "Aah" theme, but it was suggested that this be extracted.