admin: persist page view info in url parameters

- persist offset, limit, name_filter in URL parameters,
  so they are stable across page reload
- add UI element to specify items per page

This allows specifying a URL, which will show a specific view of a page of users

jsx/src/Store.js

@@ -17,6 +17,13 @@ export const reducers = (state = initialState, action) => {
         }),
       });
 
+    case "USER_LIMIT":
+      return Object.assign({}, state, {
+        user_page: Object.assign({}, state.user_page, {
+          limit: action.value.limit,
+        }),
+      });
+
     case "USER_NAME_FILTER":
       // set offset to 0 if name filter changed,
       // otherwise leave it alone