The letter chosen for the fallback site icon was being set
inconsistently throughout the extension. The connect flow was using the
first letter of the `origin` for the letter (which was always `H`,
because `HTTP`), but the connect sites list and the account menu were
using the `name` from the domain metadata.
The `name` is now used for the fallback icon everywhere. A selector
that supplied a default domain metadata object has also been augmented
to use the `hostname` rather than the `origin` as a fallback name, to
match the behavior of the inpage provider.
Mark Stacey
ec10323495