Implemented EZP-20286 - 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
* [x] Implement link generation
* [x] Finish design document
* [x] Implement high level configuration
* [x] Unit tests

## Proposed high-level configuration
```yaml
ezpublish:
    system:
        my_siteaccess:
            content:
                tree_root:
                    # Root locationId. Default is top locationId
                    location_id: 123
                    # Every URL aliases starting with those prefixes will be considered 
                    # being outside of the subtree starting at root_location.
                    # Default value is an empty array.
                    # Prefixes are not case sensitive.
                    excluded_uri_prefixes:
                        - media
                        - users
```