Use OpenLayers for maps

New pull request, since the old one was fixed to a commit id instead of a branch name, so I couldn't update it.

The code changes of the comments are mostly folded in.

TODO:
- Update configuration management (no more google API key)
- Provide support for alternative maps providers (eg. OSM), and UI to configure site defaults
- Provide optional click&point UI in metadata editor (in addition to lat/long text fields)