This enables more advanced customisation by allowing for
administrators to fully replace the footer of the login theme.
We try to take into account maintainability, at the same time mention
that it is the administrators' responsibility to keep their
customisations compatible with newer versions of DD.
This was needed because previously the DEFAULT_SERVER_URL was hardcoded
in the compiled keycloak module and that URL has changed.
For consistency sso-admin uses the same environment variables (except it
needs AVATARS_SERVER_HOST instead of AVATARS_SERVER_URL).
Evilham