MM-23489 Fix poor Unicode handling in display names (#14214)

Filter blacklisted Unicode characters from:

user: first name, last name, nickname, bot description, username
team: name, display name, description, company name
channel: name, display name

model/user.go

@@ -329,6 +329,11 @@ func (u *User) PreSave() {
 		u.AuthData = nil
 	}
 
+	u.Username = SanitizeUnicode(u.Username)
+	u.FirstName = SanitizeUnicode(u.FirstName)
+	u.LastName = SanitizeUnicode(u.LastName)
+	u.Nickname = SanitizeUnicode(u.Nickname)
+
 	u.Username = NormalizeUsername(u.Username)
 	u.Email = NormalizeEmail(u.Email)
 
@@ -362,10 +367,21 @@ func (u *User) PreSave() {
 
 // PreUpdate should be run before updating the user in the db.
 func (u *User) PreUpdate() {
+	u.Username = SanitizeUnicode(u.Username)
+	u.FirstName = SanitizeUnicode(u.FirstName)
+	u.LastName = SanitizeUnicode(u.LastName)
+	u.Nickname = SanitizeUnicode(u.Nickname)
+	u.BotDescription = SanitizeUnicode(u.BotDescription)
+
 	u.Username = NormalizeUsername(u.Username)
 	u.Email = NormalizeEmail(u.Email)
 	u.UpdateAt = GetMillis()
 
+	u.FirstName = SanitizeUnicode(u.FirstName)
+	u.LastName = SanitizeUnicode(u.LastName)
+	u.Nickname = SanitizeUnicode(u.Nickname)
+	u.BotDescription = SanitizeUnicode(u.BotDescription)
+
 	if u.AuthData != nil && *u.AuthData == "" {
 		u.AuthData = nil
 	}