feat(caldav): Add calendar timezone picker

[ChristophWurst]

https://www.rfc-editor.org/rfc/rfc4791#section-5.2.2

Description: The CALDAV:calendar-timezone property is used to
specify the time zone the server should rely on to resolve "date"
values and "date with local time" values (i.e., floating time) to
"date with UTC time" values. The server will require this
information to determine if a calendar component scheduled with
"date" values or "date with local time" values overlaps a CALDAV:
time-range specified in a CALDAV:calendar-query REPORT. The
server will also require this information to compute the proper
FREEBUSY time period as "date with UTC time" in the VFREEBUSY
component returned in a response to a CALDAV:free-busy-query
REPORT request that takes into account calendar components
scheduled with "date" values or "date with local time" values. In
the absence of this property, the server MAY rely on the time zone
of their choice.

Ref #4778 (comment)

Todo

• Make it work
• Make it pretty

[codecov]

Codecov Report

❌ Patch coverage is 0% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 22.15%. Comparing base (2692d49) to head (c96dfe9).
⚠️ Report is 2355 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/AppNavigation/EditCalendarModal.vue	0.00%	28 Missing ⚠️
src/store/calendars.js	0.00%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #4935      +/-   ##
============================================
- Coverage     22.25%   22.15%   -0.10%     
  Complexity      334      334              
============================================
  Files           227      227              
  Lines         10789    10837      +48     
  Branches       2045     2054       +9     
============================================
  Hits           2401     2401              
- Misses         8388     8436      +48     

Flag	Coverage Δ
javascript	14.15% <0.00%> (-0.08%)	⬇️
php	64.07% <ø> (ø)

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[tcitworld]

question (non-blocking): Do we need to validate the value? Maybe use calendar-js' TimezoneManager.hasTimezoneForId() before assigning it?

[SebastianKrupinski]

Closing as this is no longer relavent