MM-32133 shared channel username collisions (#17347)

Support for handling username collisions between remote clusters. Users belonging to remote clusters have their username changed to include the remote name e.g. wiggin becomes wiggin:mattermost.

@mentions are also modified so the munged username is replaced with the original username when the post is sync'd with the remote the user belongs to.

When adding remote users:
- append the remote name to the username with colon separator
- append the remote name to the email address with colon separator
- store the original username and email address in user props
- when resolving @mentions replace with the stored original username

model/user.go

@@ -272,8 +272,14 @@ func (u *User) IsValid() *AppError {
 		return InvalidUserError("update_at", u.Id)
 	}
 
-	if !IsValidUsername(u.Username) {
-		return InvalidUserError("username", u.Id)
+	if u.IsRemote() {
+		if !IsValidUsernameAllowRemote(u.Username) {
+			return InvalidUserError("username", u.Id)
+		}
+	} else {
+		if !IsValidUsername(u.Username) {
+			return InvalidUserError("username", u.Id)
+		}
 	}
 
 	if len(u.Email) > USER_EMAIL_MAX_LENGTH || u.Email == "" || !IsValidEmail(u.Email) {
@@ -746,6 +752,21 @@ func (u *User) IsRemote() bool {
 	return u.RemoteId != nil && *u.RemoteId != ""
 }
 
+// GetProp fetches a prop value by name.
+func (u *User) GetProp(name string) (string, bool) {
+	val, ok := u.Props[name]
+	return val, ok
+}
+
+// SetProp sets a prop value by name, creating the map if nil.
+// Not thread safe.
+func (u *User) SetProp(name string, value string) {
+	if u.Props == nil {
+		u.Props = make(map[string]string)
+	}
+	u.Props[name] = value
+}
+
 func (u *User) ToPatch() *UserPatch {
 	return &UserPatch{
 		Username: &u.Username, Password: &u.Password,
@@ -836,12 +857,13 @@ func ComparePassword(hash string, password string) bool {
 }
 
 var validUsernameChars = regexp.MustCompile(`^[a-z0-9\.\-_]+$`)
+var validUsernameCharsForRemote = regexp.MustCompile(`^[a-z0-9\.\-_:]+$`)
 
-var restrictedUsernames = []string{
-	"all",
-	"channel",
-	"matterbot",
-	"system",
+var restrictedUsernames = map[string]struct{}{
+	"all":       {},
+	"channel":   {},
+	"matterbot": {},
+	"system":    {},
 }
 
 func IsValidUsername(s string) bool {
@@ -853,13 +875,21 @@ func IsValidUsername(s string) bool {
 		return false
 	}
 
-	for _, restrictedUsername := range restrictedUsernames {
-		if s == restrictedUsername {
-			return false
-		}
+	_, found := restrictedUsernames[s]
+	return !found
+}
+
+func IsValidUsernameAllowRemote(s string) bool {
+	if len(s) < USER_NAME_MIN_LENGTH || len(s) > USER_NAME_MAX_LENGTH {
+		return false
 	}
 
-	return true
+	if !validUsernameCharsForRemote.MatchString(s) {
+		return false
+	}
+
+	_, found := restrictedUsernames[s]
+	return !found
 }
 
 func CleanUsername(username string) string {