Add a context menu item for jcommune re-indexing

We need to have additional menu item while pressing right button on JCommune component.
This will require some changes in the ZUL/VM because right now all the components are handled in the same way, but we need different items for different components.

• After pressing the menu item, it should send an empty HTTP request to the [jcommune_url]/search/index/rebuild.
• If a URL to the forum is not specified in the component's configuration, an error should appear and request shouldn't be send.
• Same if a wrong URL that leads to nothing was specified.