Cells: Fix cross joins in events_finder

Fix the cross joins in these files:

• app/finders/events_finder.rb:64:
• app/finders/issuables/assignee_filter.rb:9: => #417462 (closed)
• [-] app/finders/merge_requests_finder.rb:76: => #419220 (closed)
• [-] ee/app/finders/epics_finder.rb:57: => #419221 (closed)

Refer to the epic for more information about this. Feel free to:

1. Copy the epic description to this issue to helper the reviewers understand the problem.
2. Split this issue into different issues in case it was so much effort to
3. Ask for help and hints on #g_tenant-scale on how to fix cross joins