[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

app/notification.go

@@ -212,14 +212,16 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 						return
 					}
 				}
+
+				updateFollowing := *a.Config().ServiceSettings.ThreadAutoFollow
 				if mentionType == ThreadMention || mentionType == CommentMention {
 					incrementMentions = false
+					updateFollowing = false
 				}
-
 				opts := store.ThreadMembershipOpts{
 					Following:             true,
 					IncrementMentions:     incrementMentions,
-					UpdateFollowing:       *a.Config().ServiceSettings.ThreadAutoFollow,
+					UpdateFollowing:       updateFollowing,
 					UpdateViewedTimestamp: userID == post.UserId,
 					UpdateParticipants:    userID == post.UserId,
 				}