'Update' Flag for thread replies, especially for preset sharing section. Allows for 'Follow Updates' alert mode for the 'customers'

The thread starter in Setup/Preset Sharing section gains the ability to mark a new reply as an 'update', for when they update their preset. This is similar to the 'solution' Flag.

A new following mode 'Follow Updates' is automatically set for anyone who downloads. (not sure how to handle this) They will be alerted to replies marked as 'updates', similar to how Tracking works.

Only the person who started the thread can set a new reply as an 'Update'.