Extension:TimedMediaHandler/TimedTextRework

Problems:
 * It's SRT, but with wikitext.... served as HTML'ish
 * VTT has become the defacto, HTML5 captioning standard
 * Our text tracks reference action=raw, but our usage is api.php
 * We currently assume one single format, but things change

Todo:
 * Create new URL
 * Something like endpoint.php?&title=File:Example.ogv&format=[srt|vtt]&lang=en&type=[captions|subtitles|chapters]
 * would find a subtitle in a language that goes with the file
 * Allow to output as either VTT or SRT
 * This will allow us to switch more easily from SRT in TMH to VTT in VideoJS
 * Validate content. Especially VTT allows for a subset of HTML tags, that we probably should validate

Api Idea:
 * Introduce a new API module named "timed text" -> ApiTimedText
 * Module has two output formats srt and vtt -> ApiFormatSrt, ApiFormatVtt
 * Better to do it in a manner similar to how the various action=feedfoobar modules do it than pretending to have a generic formatting module:  and just ignore the global  . In this case you probably don't even need an equivalent to ApiFormatFeedWrapper, just use ApiFormatRaw.
 * Use Captioning project to convert between formats

Alternative:
 * restbase, but we need a core endpoint for that as well
 * complete new endpoint ?
 * ResourceLoader?
 * To me this feels similar to use case of serving gadget js or css sourced from pages. RL should be a good fit in theory? --brion