Fix event-following bug by adding current user following status to event serializer

[eikhr]

There is a bug where the webapp does not correctly load the following status of the current user, if an event has a large number of followers. It is caused by pagination on the event follower endpoint, but I figured this was a better solution than loading all pages in the webapp.

This adds a field "following" on EventReadUserDetailedSerializer that is false when the current user is not following the event, and the id of the relevant EventFollow object if the user is following the event.

webkom/lego-webapp#3541 implements the needed front-end changes

[codecov]

Codecov Report

Base: 88.28% // Head: 88.29% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (905c7cc) compared to base (7e08ba1).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3214   +/-   ##
=======================================
  Coverage   88.28%   88.29%           
=======================================
  Files         658      658           
  Lines       20713    20727   +14     
=======================================
+ Hits        18287    18301   +14     
  Misses       2426     2426           

Impacted Files	Coverage Δ
lego/apps/events/fields.py	90.78% <100.00%> (+0.55%)	⬆️
lego/apps/events/models.py	97.36% <100.00%> (+0.02%)	⬆️
lego/apps/events/serializers/events.py	96.77% <100.00%> (+0.01%)	⬆️

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[ivarnakken]

lgtm