Speaker selector

Addresses:
#547 
Should:
Allow the user to assign a speaker to each line of their transcript in Edit Transcript mode.
Should allow the user to edit the provided speaker names by clicking Assign Speakers.
Should allow the user to add new speakers by clicking Add Additional Speakers.

Also, it looks like I accidentally put "Remove 2-min transcript from premium" in this branch (should have gone straight into master or its own branch). That change should prevent users with premium plans from getting 2-min transcript previews.