MAde uuid column unique

ref #112

src/models/entities/RunnerOrganisation.ts

@@ -32,7 +32,7 @@ export class RunnerOrganisation extends RunnerGroup {
    * The api key can be used for the /runners/register/:token endpoint.
    * Is has to be base64 encoded if used via the api (to keep url-safety).
    */
-  @Column({ nullable: true })
+  @Column({ nullable: true, unique: true })
   @IsString()
   @IsOptional()
   key?: string;