Extension:MobileFrontend/Remove existing HTMLFormatter code from MobileFrontend extension

Motivations for removing HTMLFormatter code and possibly migrating remaining MobileFrontend logic to core

 * Reduce regressions - there have been several cases where changes to HTML have had a serious implication on mobile. For example, a change in the class name of the edit buttons in section headings resulted in a high volume of anonymous edits on mobile, many of which were spam which we had to fire fight.
 * The following bugs for example:
 * https://bugzilla.wikimedia.org/show_bug.cgi?id=48551
 * https://bugzilla.wikimedia.org/show_bug.cgi?id=48730
 * https://bugzilla.wikimedia.org/show_bug.cgi?id=50530
 * https://bugzilla.wikimedia.org/show_bug.cgi?id=51991
 * https://bugzilla.wikimedia.org/show_bug.cgi?id=52857
 * https://bugzilla.wikimedia.org/show_bug.cgi?id=52858
 * It is not sustainable. We scrub unsuitable elements in the HTML output and as a result we've had issues where edit links have shown up in sections due to HTML changes
 * Reduce code complexity: The code is a mess and a nightmare to maintain/navigate - we currently resort to a regular expression in order to have collapsible sections.
 * Performance - we shouldn't need to DOMParse the entire HTML output before rendering - we should be able to ask the parser to do these things. For example, in various places we rewrite absolute URLs to be https and use the mobile site.
 * We would like our mobile skin to work on both desktop and mobile to encourage more development effort from outside teams which currently see the existing skin as a confusing black box
 * Sharing functionality - for example we currently disable images on mobile. It would nice to share this code with users who would like to disable images on desktop.

Outcomes

 * 1) MobileFrontend would become a skin that would appear in user preferences. This would lead the way towards mobile being better integrated with core.
 * 2) It is understood that this will make integration with VisualEditor easier, although not 100% clear how.