Fix slow events query for followed users

What does this MR do and why?

#346435 (closed)

The MR fixes the slow IN query used to fetch events for followed users in Activities page using the IN operator optimization only when the event filter used is ALL (i.e., no filtering condition in the query).

Improving the IN queries with event filters requires a larger work and should be tracked separately.

Query plan comparisons

query without offset (w/ 9 followers)

• Query plan (before): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7687/commands/27306
• Query plan (after): https://explain-depesz.postgres.ai/s/bR

query with offset (w/ 9 followers)

• Query plan (after): https://explain-depesz.postgres.ai/s/cQ

Migration output

Up

== 20211217145923 AddIndexToEventsOnAuthorIdAndActionAndId: migrating =========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:events, [:author_id, :action, :id], {:name=>"index_events_on_author_id_and_action_and_id", :algorithm=>:concurrently})
   -> 0.0056s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:events, [:author_id, :action, :id], {:name=>"index_events_on_author_id_and_action_and_id", :algorithm=>:concurrently})
   -> 0.0090s
-- execute("RESET statement_timeout")
   -> 0.0012s
== 20211217145923 AddIndexToEventsOnAuthorIdAndActionAndId: migrated (0.0188s)

Down

== 20211217145923 AddIndexToEventsOnAuthorIdAndActionAndId: reverting =========
-- transaction_open?()
   -> 0.0000s
-- indexes(:events)
   -> 0.0101s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:events, {:algorithm=>:concurrently, :name=>"index_events_on_author_id_and_action_and_id"})
   -> 0.0073s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211217145923 AddIndexToEventsOnAuthorIdAndActionAndId: reverted (0.0280s) 

MR acceptance checklist

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