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

Base: 22.25% // Head: 22.15% // Decreases project coverage by -0.10% ⚠️

Coverage data is based on head (c96dfe9) compared to base (2692d49).
Patch coverage: 0.00% of modified lines in pull request are covered.

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)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/components/AppNavigation/EditCalendarModal.vue	0.00% <0.00%> (ø)
src/store/calendars.js	0.00% <0.00%> (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?