UI: useful pre-selection of calendars available for task entries

[thommierother]

Currently, tasks shows all calendars which are available for the user (private, shared though OC, remote WebCAL calendars). Moreover, "contact birthdays" is shown as a separate calendar. This makes no sense, because a click on "create new task" could lead to an unsuccessful entry into "birthday calendar".

Requirement:
Tasks should only display local OC calendars as entry points.
a) the private calendar of the user
b) shared OC calendars

"birthday calendar" should be automatically disabled (hidden).

[raimund-schluessler]

This was already discussed in #338. It is a server issue which should be fixed in oC 9.1. See owncloud/core#25636.