add discovery plugin
configure store with discovery
more flexible head slot

Using metal:fill-slot in combination with tal:omit-tag makes it easy
to provide a slot to add any number of child elements to a node. In
this case, it's used to make the <head> tag in the base template
extensible by the layout templates.

Also, make the embed deferred on the home site.
add discovery link tag
make api its own package
split oauth, flask store, and delegated store apis