IclaSignature.by_user eager loads users & accounts

Avoids accidental eager loading in IclaSignature.by_user spec
User.authorized_contributors doesn't eager load.

Remove any notation of User.authorized_contributors eager loading associated
data. Leave a NOTE suggesting against directly using associated data from users
returned.
Update logic for All Contributors tab count.

The previous implementation was not accurate. This grabs the unique count of
authorized contributors.
Make the display of All Contributors faster.

Implements the ContributorList class to eager load all accounts for the
specified users. ContributorList#each then yields a user, their chef account and
their GitHub accounts.
Write spec for the user show route.

This will alert maintainers if there is a chance the All Contributors view
breaks due to a routing change.
Sort User.authorized_contributors by Chef username