Add "Claim" button to cases

[gmittal]

A volunteer automatically claims cases (consequently preventing anyone else from accessing that case for the following hour) when they click on a case to view its messages/notes.

Add a very clear "Claim" button to the UI that allows volunteers to be very clear about when they are claiming a case. This patch would also require unclaimed cases that a volunteer browses through to have a deactivated notes and messaging box so multiple people can't message people at the same time.