feat(llc): merge feed and activity event handlers

[renefloor]

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Closes FLU-315

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Many events have impact on multiple state classes. This PR shares the type of events that are relevant for multiple state classes, so we are sure they are all handled the same.

[codecov]

Codecov Report

❌ Patch coverage is 89.38907% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.03%. Comparing base (11b075c) to head (caf6550).

Files with missing lines	Patch %	Lines
...rc/state/event/partial_activity_event_handler.dart	84.44%	7 Missing ⚠️
...ages/stream_feeds/lib/src/models/comment_data.dart	84.37%	5 Missing ⚠️
...ackages/stream_feeds/lib/src/models/poll_data.dart	78.26%	5 Missing ⚠️
...ackages/stream_feeds/lib/src/state/feed_state.dart	91.66%	5 Missing ⚠️
...stream_feeds/lib/src/state/comment_list_state.dart	75.00%	4 Missing ⚠️
...ate/event/partial_activity_list_event_handler.dart	91.83%	4 Missing ⚠️
..._feeds/lib/src/state/event/feed_event_handler.dart	83.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   40.86%   48.03%   +7.17%     
==========================================
  Files         120      122       +2     
  Lines        3617     3818     +201     
==========================================
+ Hits         1478     1834     +356     
+ Misses       2139     1984     -155     

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