MM-17071 Add mention counting when marking a post as unread (#11966)

* Add different types for different mentions

* Remove redundant THREAD_ANY and THREAD_ROOT constants

* Make PostStore.Get return thread in order

* MM-17071 Add initial version of countMentionsFromPost

* MM-17071 Refactor comment mention counting

* MM-17071 Use mention counting when marking post as unread

* Fix shadowing in tests

* Remove repeated check of user count

* Refactor code using MentionType

* Update comments around -1 return value

* Move inner functions out of countMentionsFromPost

* Remove preconditions check as separate test case

* Update comments

* Add User.GetMentionKeys

* Revert "Make PostStore.Get return thread in order"

This reverts commit 22aa010cee359655fe75e1dc899cf0ffb0943c2a.

* Fix tests

* Fix merge conflict

* Add store.MentionAllPosts

model/user.go

@@ -397,8 +397,7 @@ func (u *User) SetDefaultNotifications() {
 
 func (user *User) UpdateMentionKeysFromUsername(oldUsername string) {
 	nonUsernameKeys := []string{}
-	splitKeys := strings.Split(user.NotifyProps[MENTION_KEYS_NOTIFY_PROP], ",")
-	for _, key := range splitKeys {
+	for _, key := range user.GetMentionKeys() {
 		if key != oldUsername && key != "@"+oldUsername {
 			nonUsernameKeys = append(nonUsernameKeys, key)
 		}
@@ -410,6 +409,22 @@ func (user *User) UpdateMentionKeysFromUsername(oldUsername string) {
 	}
 }
 
+func (user *User) GetMentionKeys() []string {
+	var keys []string
+
+	for _, key := range strings.Split(user.NotifyProps[MENTION_KEYS_NOTIFY_PROP], ",") {
+		trimmedKey := strings.TrimSpace(key)
+
+		if trimmedKey == "" {
+			continue
+		}
+
+		keys = append(keys, trimmedKey)
+	}
+
+	return keys
+}
+
 func (u *User) Patch(patch *UserPatch) {
 	if patch.Username != nil {
 		u.Username = *patch.Username