feat(webcal): Name button *Subscribed* for existing calendars

[ChristophWurst]

Fixes #5192

Before	After

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.04 ⚠️

Comparison is base (12deb19) 22.46% compared to head (d66843d) 22.43%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5195      +/-   ##
============================================
- Coverage     22.46%   22.43%   -0.04%     
  Complexity      373      373              
============================================
  Files           237      237              
  Lines         11575    11573       -2     
  Branches       2176     2176              
============================================
- Hits           2600     2596       -4     
- Misses         8975     8977       +2     

Flag	Coverage Δ
javascript	13.90% <ø> (-0.04%)	⬇️
php	63.94% <ø> (ø)

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Impacted Files	Coverage Δ
...ponents/Subscription/HolidaySubscriptionPicker.vue	0.00% <ø> (ø)

... and 1 file with indirect coverage changes

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