Not allowing to use team invite links by guest accounts (#12608)

* Not allowing to use team invite links by guest accounts

* Adding needed tests

* Updating error text

api4/team.go

@@ -499,6 +499,11 @@ func addUserToTeamFromInvite(c *Context, w http.ResponseWriter, r *http.Request)
 	var member *model.TeamMember
 	var err *model.AppError
 
+	if c.App.Session.Props[model.SESSION_PROP_IS_GUEST] == "true" {
+		c.Err = model.NewAppError("addUserToTeamFromInvite", "api.team.add_user_to_team_from_invite.guest.app_error", nil, "", http.StatusForbidden)
+		return
+	}
+
 	if len(tokenId) > 0 {
 		member, err = c.App.AddTeamMemberByToken(c.App.Session.UserId, tokenId)
 	} else if len(inviteId) > 0 {