Added parameter to hide option for calendar subscription via link

Added calendar application config parameter `allow_subscribe_link` to hide option for
calendar subscription via link  (i.e. for internal Nextcloud setups). Use

```
occ config:app:set calendar allow_subscribe_link --value 'yes'
```

to show (default if not set) and

```
occ config:app:set calendar allow_subscribe_link --value 'no'
```

to hide calendar subscription via link option. Should be applied together with
https://github.com/nextcloud/server/pull/34387

Related: https://github.com/nextcloud/server/pull/34387


Author-Change-Id: IB#1126265
Signed-off-by: Pawel Boguslawski <pawel.boguslawski@ib.pl>