[MM-36544][MM-37439] Don't re-follow on reply to unfollowed thread (#18020)

Summary
If a user has unfollowed a thread, another user's reply in the thread should not cause the first user to re-follow the thread. The first user will be re-followed if they are mentioned in the thread.
Simplify and make flexible the logic surrounding following and remove some duplicate code.

Ticket Link
https://mattermost.atlassian.net/browse/MM-36544
https://mattermost.atlassian.net/browse/MM-37439

store/store.go

@@ -934,7 +934,7 @@ type ThreadMembershipOpts struct {
 	// IncrementMentions indicates whether or not the mentions count for
 	// the thread should be incremented.
 	IncrementMentions bool
-	// UpdateFollowing indicates whether or not a membership update should be forced.
+	// UpdateFollowing indicates whether or not the following state should be changed.
 	UpdateFollowing bool
 	// UpdateViewedTimestamp indicates whether or not the LastViewed field of the
 	// membership should be updated.