feat(caldav): create subscriptions via occ

[miaulalala]

Summary

Allow adding subscriptions via OCC

TODO

• Docs

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

Inject the CalDavBackend to get rid of all the service locators

[ChristophWurst]

Looks good

[tcitworld]

Could be nice to execute subscription sync to test is the URL is valid?

[miaulalala]

Could be nice to execute subscription sync to test is the URL is valid?

I did have that initially, but since it's quite a lot of code duplication I desisted from doing that as it's not really neccessary. The RefreshWebCal Job will log if the url isn't valid.