[server] Prevent changing role for teams of size 1

Author: easyCZ

Diff for: components/gitpod-db/src/typeorm/team-db-impl.ts

@@ -203,6 +203,12 @@ export class TeamDBImpl implements TeamDB {
             throw new ResponseError(ErrorCodes.NOT_FOUND, "A team with this ID could not be found");
         }
         const membershipRepo = await this.getMembershipRepo();
+
+        const teamSize = await membershipRepo.count({ teamId, deleted: false });
+        if (teamSize <= 1) {
+            throw new ResponseError(ErrorCodes.BAD_REQUEST, "Cannot change role on the last remaining team member");
+        }
+
         const membership = await membershipRepo.findOne({ teamId, userId, deleted: false });
         if (!membership) {
             throw new ResponseError(ErrorCodes.NOT_FOUND, "The user is not currently a member of this team");