[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/sqlstore/thread_store.go

@@ -611,7 +611,7 @@ func (s *SqlThreadStore) MaintainMembership(userId, postId string, opts store.Th
 	// b. mention count changed
 	// c. user viewed a thread
 	if err == nil {
-		followingNeedsUpdate := (opts.UpdateFollowing && !membership.Following || membership.Following != opts.Following)
+		followingNeedsUpdate := (opts.UpdateFollowing && (membership.Following != opts.Following))
 		if followingNeedsUpdate || opts.IncrementMentions || opts.UpdateViewedTimestamp {
 			if followingNeedsUpdate {
 				membership.Following = opts.Following

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.