[WIP] Multisite with single content repository

Jira issue : https://jira.ez.no/browse/EZP-20286

This PR addresses the issue regarding the possibility to build multiple websites with a single content repository, in different subtrees of content.

The main idea is to gather the 3 legacy settings (`PathPrefix`, `IndexPage`, `RootNode`) into one, `content.root_location`.

## Tasks
* [x] Override the `UrlAliasRouter` to take the root location as a starting point for routing
* [ ] Implement link generation
* [ ] Finish design document

An open question is about the [`PathPrefixExclude` legacy setting](https://github.com/ezsystems/ezpublish-legacy/blob/master/settings/site.ini#L745). Keeping  it is arguable since it appears to be a *hack* of the initial feature ([see original issue](https://jira.ez.no/browse/EZP-8003)).
My opinion is that if you want to have a root location, you expect to have everything under it (routing wise).
However, getting rid of this setting would still let you fetch locations outside of your subtree via API or sub-requests. You just won't be able to make direct links to those. Thoughts ?