AbuseFilterViewDiff: Make diffs use standard styling (and more)

They still used old 1.18-style diffs, with styles copied from core.

Also:
* Remove the distinction between "simple" and "multiline" diff chunk,
  enabling consistent styling
* Always show word-level diff, even in previously "simple" chunks
* Ensure the full context is always shown. This was clearly intended
  (showing straight side-by-side comparison if there were no changes
  and in "simple" chunks, diff otherwise; showing chunks with no
  changes), but the default context is only 2 lines.
* Remove the distinction between 'wikitext' and 'text' diffs (all are
  treated like 'text' was); it's impossible to show parsed code with
  word-level diff, and it was only enabled for "simple" chunks, all of
  which are intended to be one-line strings and are shown unparsed
  elsewhere (e.g. filter description on the list on Special:AbuseFilter)

Bug: 36478
Change-Id: Ie24ad5ac5a9a2bc9b8f0fc0c4a03d5dfe7a6a83f