Snapshot service

This PR adds a first version of a snapshot service. The snapshort service will create a snapshot of the site specified with the geoadminhost variable.

It will be used to serve static pages for web crawlers. It uses selenium with PhantomJS to create the static version. It relies on https://github.com/geoadmin/mf-geoadmin3/pull/1123 to be applied to the geoadminhost for it to work properly.

This is safe to be merged as it doesn't impact anything else and crawlers are not re-directed to this service yet.

Note: there's only basic error handling.
Note: we need to load test this service in order to be sure that crawlers will not kill our instances
Note: I'll document the basic SEO process in a swimlane diagram