Add Fronts section to Football admin tool

This will allow editors to choose football components for inclusion on
fronts via the Facia tool.
Add preview at the various fronts component sizes
Improve rendering of Football Fronts tool previews
House keeping in football admin

Remove some hard-coded nasties from the admin tool
Tidy up FrontsController and add facia snap fields to embed links
Add Fronts to the football admin's nav
Football admin front embed fixes

Improve preview and more intelligently work out the host to use for
the previews and facia embed link.
Nothing to see here
Update config test and tidy some imports
PR tidy

* remove reference to internal name, Facia
* use safer Option constructor instead of Some
Merge pull request #4045 from guardian/fronts-components-in-admin

Fronts components in admin