Forbid to locally edit tasks in read-only calendars

[raimund-schluessler]

A user can change properties of tasks in read-only calendars with v0.11.0. Although changes are not written to the server (as it should be), this is confusing for the user and shouldn't be possible. See #72 (comment).

Things that should not be possible, but are at the moment:

• In the list views: completing and starring a task
• In the details view: Completing and starring a task, changing the calendar and classification and adding tags

Also, when a property is not set, e.g. no start or due date is set, the field shouldn't be shown at all (since no information is shown).

[raimund-schluessler]

Also, the option to delete all completed tasks should not be shown for read-only calendars.