Add protected labels

[osteele]

This is an umbrella issue for the addition of protected fields:

• Add a (proposed) protected field to Label.
• A client must not be able to create an event with a protected tag.
• A client must not be able to modify an event to add a protected tag.
• A client must not be able to create an ICS subscription with a protected tag.
• A client must not be able to modify an ICS subscription to include a protected tag.

#218 covers event protection. #240 covers ICS subscription protection.

Additional work for this task includes:

This task includes the proposal of a set of protected tags. (StAR; maybe Academic Calendar; others?)