Extension:TimedMediaHandler/2018-2019 work

Current rough plans for the 2018-2019 season's work on TimedMediaHandler:

Immediate (Q1)
Feature track
 * subtitle API with lax runtime conversion from SRT to VTT ✅
 * finish old subtitle API patch updated with new conversion engine
 * deploy, or prepare, deployment of video.js ✅
 * remove old kaltura player bits from TMH ❌

Tech track
 * switch to VP9 transcodes (smaller file sizes for many, better quality for most) ✅
 * merging final patches, then ready to start any time!
 * update ogv.js to latest release (minor VP9 fixes) ✅
 * tweak ogv.js thread management to handle large number of paused players better ✅

Next steps (Q2-Q3)
Feature track
 * customize frontend controls for videojs-mode players ❌
 * inline start/stop ✅
 * easy expansion to zooming player into MediaViewer ✅
 * get video playback working in iOS Wikipedia mobile app ❌
 * either tidy up OGVKit or ship the ogv.js player embedded
 * clean up reporting tools for transcode errors, queue state (Special:TimedMediaHandler, File: page table) ❌
 * experiment with frontend UI for on-wiki audio/video trimming/rotation manipulation tool ❌
 * experiment with frontend UI for on-wiki subtitle editing tool ❌

Tech track
 * ogv.js work for MPEG-DASH adaptive streaming ❌
 * add MSE interfaces to ogv.js ❌
 * get video.js's DASH talking to ogv.js
 * ffmpeg research for MPEG-DASH adaptive streaming
 * create webm vp9 file-per-track streams and play them back with video.js
 * create webm vp9 chunked streams and play them back with video.js
 * create webm vp9 chunked streams through multiple separate ffmpeg invocations and stitch the manifests together (desired for better job queue management)
 * research on-wik trimming rotation / manipulation tool ❌
 * conversions done with ffmpeg command options

Medium future (Q4)
Feature track
 * experiment with on-wiki trimming / rotation manipulation tool ❌
 * experiment or deploy on-wiki subtitle editing tool ❌

Tech track
 * (optional) limit flat files to lower resolution to save space ❌
 * deploy MPEG-DASH adaptive streaming to supplement existing flat files ❌

Farther future
Feature track
 * re-evaluate more advanced video manipulation and editing

Tech track
 * evaluate AV1 compression & playback for eventual supplement/replacement of VP9 ❌
 * evaluate MP4 H.264, HEVC, AAC ingest ❌
 * set up MP4 H.264 output options for compatibility if license issues ever resolved