Fix #316 : hide git-notes

The solution adopted for the network visualization is just to remove all nodes with no branch. This cleans out notes, and might also crean other "orphan" nodes, as could be the case for nodes whose head was lost due to a forced push, for example.

For the dashboard, it just doesn't consider events associated to notes (refs/notes). A better solution might be to patch the event creation, although that would require also some cleaning on the events table.