Editor for WebVTT files

[kstapelfeldt]

Users can't edit cues in the player when they come from VTT source files (as apposed to the transcript XML format) but it looks like they can, until they encounter the error "TRANSCRIPT does not have a valid xml content" - ideally we'd remove the edit button when transcripts were provided as VTT or provide an alternate editor.

[kstapelfeldt]

For now, we will remove edit button for VTT - there are issue here related to our support of two types of source files and the derivatives we create, and the necessity of keeping derivatives synced with source files. Workflow needs to be updated and this needs to be documented.