This was intended to provide a better UX for interactive elements in the
composer preview. However, the morphing strategy has irreconcilable
conflicts with our `decorateCooked` API, and so we have been unable to
enable this by default.
Going forward, we're focussing efforts on the WYSIWYG composer to
provide this kind of smooth UX, so we're dropping the
`enable_diffhtml_preview` approach.
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.
This allows text editors to use correct syntax coloring for the heredoc sections.
Heredoc tag names we use:
languages: SQL, JS, RUBY, LUA, HTML, CSS, SCSS, SH, HBS, XML, YAML/YML, MF, ICS
other: MD, TEXT/TXT, RAW, EMAIL
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
