Don't link to calendar, if calendar is not enabled/available

[SystemKeeper]

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Even if calendar is not available, we still link to calendar on the dashboard:

but then you end up on a "not found" page:

[Antreesy]

API:
Need to expose that calendar is enabled via capabilities (so Desktop client is aware)

Frontend:
There are two places with generateUrl('apps/calendar') and two with eventRoom.eventLink