Add Timed Transcripts Editor.

This PR adds timed transcripts editor functionality for video player in studio.

Design wireframes for this PR is [on wiki] (https://edx-wiki.atlassian.net/wiki/pages/viewpage.action?pageId=28049432)

Documentation of transcripts workflow is placed inside this PR to developers documentations folder.
Please navigate to sphinx generated developers documentation -> CMS->Transcripts

@lyla, @frrrances, @singingwolfboy please review.

You can play with it on Blades sandbox at http://studio.sandbox-blades-001.m.edx.org/edit/i4x://private/05/vertical/f1d269cde2784fa99cd5e0f3a9604d62 .

Rollback functionality is not part of this PR.