* 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.
