app: Add a way to manage a user's group memberships

app/queries/add_user_to_group.graphql

@@ -0,0 +1,5 @@
+mutation AddUserToGroup($user: String!, $group: Int!) {
+  addUserToGroup(userId: $user, groupId: $group) {
+    ok
+  }
+}

app/queries/get_group_list.graphql

@@ -0,0 +1,6 @@
+query GetGroupList {
+  groups {
+    id
+    displayName
+  }
+}

app/queries/get_user_details.graphql

@@ -7,6 +7,7 @@ query GetUserDetails($id: String!) {
     lastName
     creationDate
     groups {
+      id
       displayName
     }
   }

app/queries/remove_user_from_group.graphql

@@ -0,0 +1,5 @@
+mutation RemoveUserFromGroup($user: String!, $group: Int!) {
+  removeUserFromGroup(userId: $user, groupId: $group) {
+    ok
+  }
+}