Move Group mutations to entities

app/soapbox/hooks/api/groups/useJoinGroup.ts

@@ -0,0 +1,20 @@
+import { Entities } from 'soapbox/entity-store/entities';
+import { useEntityActions } from 'soapbox/entity-store/hooks';
+import { groupRelationshipSchema } from 'soapbox/schemas';
+
+import type { Group, GroupRelationship } from 'soapbox/schemas';
+
+function useJoinGroup(group: Group) {
+  const { createEntity, isLoading } = useEntityActions<GroupRelationship>(
+    [Entities.GROUP_RELATIONSHIPS, group.id],
+    { post: `/api/v1/groups/${group.id}/join` },
+    { schema: groupRelationshipSchema },
+  );
+
+  return {
+    mutate: createEntity,
+    isLoading,
+  };
+}
+
+export { useJoinGroup };