Use timezone ID instead of full VTIMEZONE in DB

[ArnyminerZ]

Purpose

Currently we are storing the full VTimeZone definition in the database, even though we only use the timezone ID. This PR aims to fix this, and only store the identifier.

Short description

• Removed the column timezone from Collection
• Added a new column timezoneId to Collection
• Upgraded database version number to 15
• Added migrations for database from 14 to 15:
  • Create the new timezoneId column.
  • Extract the timezone id from timezone, and copy it to timezoneId (if not null)
  • Remove the timezone column.

[!NOTE] We are storing the timezone id as given by the server, which may not be a valid Android Timezone. This is obviously converted later on when storing into the calendar provider, but maybe it's not a bad idea to convert it beforehand.

More information

• calendar-timezone-id is already being used for MKCALENDAR since https://github.com/bitfireAT/davx5-ose/pull/1044
• dav4jvm got support for CalendarTimezoneId at https://github.com/bitfireAT/dav4jvm/commit/fbd95a5f5a94392d48a27f502cdad0ae45fa3932
• calendar-timezone must contain a full CALENDAR as speciifed by Calendaring Extensions to WebDAV (CalDAV) Section 5.2.2
• calendar-timezone-id definition: CalDAV: Time Zones by Reference - Section 5.2

Checklist

• [x] The PR has a proper title, description and label.
• [x] I have self-reviewed the PR.
• [x] I have added documentation to complex functions and functions that can be used by other modules.
• [ ] I have added reasonable tests or consciously decided to not add tests.

[rfc2822]

Is there something missing for this PR yet?

[ArnyminerZ]

Is there something missing for this PR yet?

It should be ready, I forgot to mark it as complete 😅