Replace + fab with unlock icon for sessions that aren't the latest

[simonalveteg]

To prevent accidental edits older sessions should be locked unless the unlock fab is clicked (with an 'are you sure' dialog?). Only the most recent session should be unlocked by default.

Should prevent exercises from being expanded as well.

[simonalveteg]

Only sessions that haven't had their end time set should be unlocked? Might be a better implementation.

Should end time be set to 0 for new sessions? Currently set to start time, but if start time is changed they aren't the same anymore which makes it harder to tell if end time has been set or not