Adds support for self-hosted GitHub Enterprise and GitLab instances. The
extension previously only ran on github.com, gist.github.com, and gitlab.com.

Users can now enable an origin from the popup by picking a platform. Each
origin is requested individually at that moment via optional permissions —
nothing is requested at install time, and the install prompt is unchanged.
Revoking the permission in browser settings disables it.

New permissions: "storage" holds the origin-to-platform mapping.
"optional_permissions: :///*" is the ceiling for what a user may later
grant, one origin at a time.

Better async handling