More than one person can work on the same transcription

[marw12]

we can ensure that more than one user doesn't work on the same transcription at the same time by having a feature that prevents users from making any changes while a user is already working on it.

[rsmithlal]

As we discussed today, we are changing our approach for this. We are now going to allow multiple people to work on the same page, and show the annotations created by others in grey. #131