Move to a scope-based host find

* In anticipation of eager-loading site and organisation
* Slight performance boost due to only looking up the host
  once and not hitting the AR query cache the second time

Before:

vagrant@development:/var/govuk/bouncer$ rake benchmark:all
              user     system      total        real
no host:  1.070000   0.090000   1.160000 (  1.298318)
410:      2.870000   0.060000   2.930000 (  3.204892)
301:      2.090000   0.030000   2.120000 (  2.348241)
404:      2.540000   0.040000   2.580000 (  2.818832)

After:

vagrant@development:/var/govuk/bouncer$ rake benchmark:all
              user     system      total        real
no host:  1.140000   0.090000   1.230000 (  1.403091)
410:      2.450000   0.050000   2.500000 (  2.790347)
301:      1.730000   0.010000   1.740000 (  1.976360)
404:      2.180000   0.040000   2.220000 (  2.491334)
Started to attempt to eager load just :site.

Note that the tests don't pass here because they're mock-heavy and brittle,
and it's not worth the effort to change them, because:

Performance disaster:

vagrant@development:/var/govuk/bouncer$ rake benchmark:all
              user     system      total        real
no host:  2.470000   0.050000   2.520000 (  2.848202)
410:      2.910000   0.040000   2.950000 (  3.399709)
301:      2.010000   0.000000   2.010000 (  2.366513)
404:      2.550000   0.020000   2.570000 (  3.012763)