Read-only collections: allow local changes?

[rfc2822]

Currently, we don't allow local changes (like the alarm time, which can be edited for otherwise read-only events in some calendar apps) in read-only collections:

• Locally modified events are reset (Don't upload event when calendar is "read-only" #572), so that a modified alarm will be reset, too
• Locally modified tasks of read-only calendars are not handled yet and will probably cause a 403 error (Don't upload locally modified tasks in read-only task lists #599)

I wonder whether we want to allow local changes in read-only events. This would allow users to still edit for instance a local event's alarm. Sync could just reset the dirty flag. Only when the ETag on the remote server changes, it would be downloaded again.

(Discussion please in #601)