feat: Add link to the Thunderbird holiday calendar page

[ChristophWurst]

This is the MVP version of #962. We plan to add an advanced modal to our app later so users don't have to leave the page.

Before	After

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (e0f778c) 22.59% compared to head (5f071dd) 22.58%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5166      +/-   ##
============================================
- Coverage     22.59%   22.58%   -0.01%     
  Complexity      373      373              
============================================
  Files           236      236              
  Lines         11475    11480       +5     
  Branches       2149     2152       +3     
============================================
  Hits           2593     2593              
- Misses         8882     8887       +5     

Flag	Coverage Δ
javascript	14.01% <0.00%> (-0.01%)	⬇️
php	63.92% <ø> (ø)

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Impacted Files	Coverage Δ
...nts/AppNavigation/CalendarList/CalendarListNew.vue	0.00% <0.00%> (ø)
src/components/AppointmentConfigModal.vue	0.00% <0.00%> (ø)
...s/AppointmentConfigModal/CheckedDurationSelect.vue	0.00% <ø> (ø)
...rc/components/AppointmentConfigModal/TextInput.vue	0.00% <0.00%> (ø)
.../fullcalendar/localization/dateFormattingConfig.js	0.00% <ø> (ø)
src/models/appointmentConfig.js	0.00% <0.00%> (ø)
src/store/settings.js	87.71% <0.00%> (ø)

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