Upgrade to Rouge 2.0

## What does this MR do?

This MR is a refactor of the HTMLGitlab formatter. The original was copy-pasted from the legacy HTML formatter and adapted to suit our needs. In particular, this MR:

* Strips dead code and unused options
* Factors out the "wrapping" into the places that care about it - Bonzai and the view helpers
* Uses the Rouge 2.0 `token_lines` method to split tokens into lines, removing the custom code we used to do the same
* Uses the now-public `span` method instead of re-implementing it ourselves
* Removes options to not split into lines - the places where this feature wasn't being used are now using the plain `Formatters::HTML` instead of this class

See merge request !4691