WiP on selectable mosaico templates.
TODO: Make files always point to trusted URL, such that we don't have to rebase them. They are public anyway. The same goes for mosaico endpoints: /mosaico/templates and /mosaico/img
Reworked routing and breadcrumb mechanism. It allows resolved parameters in paths, which allows including names of entities in the breadcrumb.
Secondary navigation which is aware of permissions.
Added support for ajax-based server side validation (useful for validation of emails, duplicate usernames, etc.)
User form more or less ready in the basic version (i.e. without permission management)
