* Added keyword arguments to truncated_diff_lines method to allow for using highlighting or not (html templates vs. text)
* Tweaked templates for consistency and format appropriateness

Previously the `truncated_diff_lines` method for outputting a discussion diff took in already highlighted lines, which meant it wasn't reuseable for truncating ANY lines. In the way it was used, it also meant that for any email truncation, the whole diff was being highlighted before being truncated, meaning wasted time highlighting lines that wouldn't even be used (granted, they were being memoized, so perhaps this wasn't that great of an issue). I refactored truncation away from highlighting, in order to truncate formatted diffs for text templates in email, using `>`s to designate each line, but otherwise retaining the parsing already done to create `diff_lines`.

Additionally, while notes on merge requests or commits had already been tested, there was no existing test for notes on a diff on an MR or commit. Added mailer tests for such, and a unit test for truncating diff lines.

Currently comments on commits and merge requests do not require merge request- or commit-specific information, but can use the same template. Rather than change the method which calls the template, I opted to keep the templates separate and create a new template to highlight their identicality, while preserving the option to distinguish them from each other in the future.

Also removed some of the inconsistencies between text and html email versions.

Still needed is a text-only version of git diffs and testing.

Every email has a different way of showing a link to the discussion on
the website. We don't need this anymore, as we now have a standard
"View in GitLab" link in the footer of every email.

Didn't bother with files in db/, config/, or features/

Update method to take ids and then perform #finds itself during mailer
queue worker kick-off. Also, the faux SHA1 cannot have underscores or
it will not match the commit pattern defined in the routes.