Not 100% sure why the changes in `PrettyText.format_for_email` raised this issue, but we were missing adding a link to the Discourse instance whenever we are replacing the elided part of a post with a link to either the post or the Discourse instance in the email.
Also reformated the specs using better variable names (sometimes a variable named `md` would contain some html) and used the `match_html` helper for all the tests.
Most of the core plugins were already hidden, this hides
chat, styleguide, and checklist to avoid potential confusion
for end users.
Also removes respond_to? :hide_plugin, since that API has been
in place for a while now.
Per the linked site (http://mths.be/details) and https://caniuse.com/#feat=details the <details> element is supported since Chrome 12/Safari 6/Firefox 49/Edge 79. IE 11 doesn't handle them, but we dropped the support for that browser.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
