Added comment to getUserMedia call in photocapture.js
Doc: update xblock publish() function signature
Add in metric tracking to xqueue.

Covers external grading and certificates.
BLD-950: Update captions logic on front-end.
Merge pull request #3128 from edx/anton/transcript-generation

Update captions logic on front-end.
Merge pull request #3141 from edx/db/doc-xblock-signature

Doc: update xblock publish() function signature
Merge pull request #3138 from edx/db/add-getusermedia-note

Added comment to getUserMedia call in photocapture.js
Merge pull request #3144 from edx/diana/monitor-xqueue-graders

Add in metric tracking to xqueue.
HD control is disabled initially. It eventually gets enabled on first play of video, when player.getAvailableQualityLevels returns on non-empty array. [BLD-937]
Addressed comments.