Fix activation of shared calendars.

Editing shared calendars deactivates them and it is impossible to
activate them afterwards.

There are actually two problems:
- Shared calendars are deactivated when editing because they are
  missing their checkbox. Consequently, the checked status can not
  be read and the update function sets it to 0.
- As shared calendars do not have a checkbox, it is not possible to
  toggle their activation state afterwards.

This change adds a checkbox for shared calendars and allows their
activation, fixing both problems.