The existing implementation used a ErrorResponse method to propogate
and create http error messages. Since we added functionality to
troubleshoot, signout, and do other tasks following an http error
it's useful to use Error struct in place of method arguments.
This fixes#157 where a troubleshooting links were appearing on pages
that it didn't make sense on (e.g. pages without valid sessions).
internal/sessions: session state is domain scoped
internal/sessions: infer csrf cookie, route scoped
proxy & authenticate: use shared cookie name
proxy & authenticate: prevent resaving unchanged session
proxy & authenticate: redirect instead of error for no session on login
internal/config: merge cookies
proxy: remove favicon specific route
proxy: use mock server for tests
proxy: add tests for failures
