UX: Add notice when watched words are regexes (PR #13493)

There is a big difference between regular watched words and regular expressions and this has been confusing in the past. This notice adds an explanation.

This commit also reorganizes the code of the test modal.


This pull request has been mentioned on Discourse Meta. There might be relevant details there:

        regex_warning: '<a href="%{basePath}/admin/site_settings/category/all_results?filter=watched%20words%20regular%20expressions%20">Watched words are regular expressions</a> and they do not automatically include word boundaries. If you want the regular expression to match whole words, include <code>\b</code> at the start and end of your regular expression.'