Add a Shareabouts project section

**New dependencies:**
- Leaflet
- Shareabouts.js (gh-pages branch)
- gatekeeper (from the OpenPlans GitHub account)

**Migrations:**
- Add an `active` field to sections
- Add a Shareabouts section to the default template

**New environment variables:**
- `SHAREABOUTS_HOST` -- the host name (e.g. *api.shareabouts.org*) for the Shareabouts data setver
- `SHAREABOUTS_USERNAME` -- The username of the planbox user on the API server
- `SHAREABOUTS_PASSWORD` -- the password for the planbox user