Add unique referer IDs in the back end
Also add the referer ID to the navigation menu links
Preserve the table-specific referers
Fall back to the latest entry if there is no matching key
Limit the fallback routine to the back end
Optimize the session data

Only store the first eight characters of any MD5 hash and never include
the relative path to the installation (`TL_PATH`), because it will be stripped
anyways in `System::getReferer()`.
Update the referer ID of links loaded via Ajax
Merge branch 'feature/referer-tab-support' into develop

Conflicts:
	assets/contao/js/core.js