Add public activity series

[nicksellen]

Backend PR karrot-dev/karrot-backend#1285

Currently Karrot supports public activities for individual activities but not public activity series.

This would help as it saves people from having to manually set each activity in a series to public.

TODO

• review frontend
• review backend
• decide how it should series setting should relate to activity setting (maybe only allow setting in the series...?)
• decide how disabling an activity should work for public activities (hide it publically? or show it as disabled?)

[codecov]

Codecov Report

Attention: 138 lines in your changes are missing coverage. Please review.

Comparison is base (e61ce32) 73.91% compared to head (34919e7) 73.72%.

Files	Patch %	Lines
src/groupInfo/pages/PublicActivitiesEmbed.vue	0.00%	131 Missing ⚠️
src/activities/components/ActivityEdit.vue	55.55%	4 Missing ⚠️
src/groupInfo/pages/GroupPreview.vue	66.66%	2 Missing ⚠️
src/activities/pages/PublicActivity.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2701      +/-   ##
==========================================
- Coverage   73.91%   73.72%   -0.19%     
==========================================
  Files         404      406       +2     
  Lines       44096    44309     +213     
  Branches     2451     2455       +4     
==========================================
+ Hits        32593    32668      +75     
- Misses      11503    11641     +138     

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