About Modal

Add an about modal that pings "/v2/info" for this Marathon instance's configuration.

Add more keyboard nav:

* "g a": go to apps
* "g d": go to deployments
* "shift+,": open about modal
Should we maybe switch around these two? I'd like to have About closest to the rest of the menu items (because it has an internal function), whereas the docs opens an external page. On the other hand I think docs is of higher priority. Should we maybe use a button, since it is not an actual navigation? I'm not sure about this.