Fix TypeORM errors for nullable fields

For fields with '| null' in the TS type, the column type needs to be
specified explicitly in column options when using MySQL.

See:
https://github.com/typeorm/typeorm/issues/1358#issuecomment-391736766

backend/src/data/entities/AllowedGuild.ts

@@ -9,7 +9,7 @@ export class AllowedGuild {
   @Column()
   name: string;
 
-  @Column({ nullable: true })
+  @Column({ type: String, nullable: true })
   icon: string | null;
 
   @Column()