Add public activity series

[nicksellen]

Backend PR https://github.com/karrot-dev/karrot-backend/pull/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

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

[codecov[bot]]

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 :warning:
src/activities/components/ActivityEdit.vue	55.55%	4 Missing :warning:
src/groupInfo/pages/GroupPreview.vue	66.66%	2 Missing :warning:
src/activities/pages/PublicActivity.vue	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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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