Add estimated time to transcribe

[tamsingreen]

For approximate transcribing time, we have talked about using the average transcribing time of previous media files in the database. It is proposed we calculate this using:
runtime - in milliseconds (time to transcribe) - from /projects/${id}/transcripts
duration - in seconds (duration of media) - from /users/${user}/uploads
speed (s) = (runtime/1000) / duration
Take the 5(?) most recent jobs, calculate speed then average them
estimated time to transcribe(s) = average * media duration
Store estimated time to transcribe in the db in seconds and human readable form

Create this in a function and run it when a transcript is requested so we can display to users how long roughly their transcript will take.

See design here: https://app.zeplin.io/project/5ce554083480691e05af2f74/screen/608bd86820f3fe2fd5d4f67a
(third transcription item down)