MM-34609 Mark-as-unread on a post in a thread should cause auto-follow (#17343)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/store.go

@@ -272,7 +272,7 @@ type ThreadStore interface {
 	GetMembershipsForUser(userId, teamID string) ([]*model.ThreadMembership, error)
 	GetMembershipForUser(userId, postID string) (*model.ThreadMembership, error)
 	DeleteMembershipForUser(userId, postID string) error
-	MaintainMembership(userID, postID string, following, incrementMentions, updateFollowing, updateViewedTimestamp bool) error
+	MaintainMembership(userID, postID string, following, incrementMentions, updateFollowing, updateViewedTimestamp bool) (*model.ThreadMembership, error)
 	CollectThreadsWithNewerReplies(userId string, channelIds []string, timestamp int64) ([]string, error)
 	UpdateUnreadsByChannel(userId string, changedThreads []string, timestamp int64, updateViewedTimestamp bool) error
 }