event.add_organizer needs some TLC

[tobixen]

The test coverage report uncovered some obscure method add_organizer that was part of my efforts on scheduling some years ago. It has the following problems:

• Function is not covered by tests
• Function possibly does wrong things if the organizer field already exists
• Function can only add the current principal to the organizer field - this contrasts with the name of the method. Should take optional parameter organizer (email string or Principal object)

[tobixen]

Depends on #596. Time estimate: 6 hours (two hours on each bullet point)

[tobixen]

All the flagged items have been fixed in the scheduling-testing branch (PR #646).