Add 'managed_by_transition' flag to Site

This flag defaults to true so that new sites added to the Transition
app first will be automatically managed there. The flag is set to
false by the migration for all existing sites, because at the moment
all sites are still managed by Redirector.

The flag represents the intended state of the world; requests will
not actually be sent to Redirector or Bouncer until the CDN has been
configured for each host, according to its site's flag. A name for
this column indicating that it reflects intention rather than reality
would be inconveniently cumbersome, and hopefully the delay between
changing the flag and the CDN configuration being updated will be
small in practice.
Add basic hosts API

This provides enough data to use to configure the CDN automatically.
Don't require authentication for hosts API

Also add tests for hosts controller.
Add an aka_hostname method to Host

We don't want or need to store the aka hostnames as separate hosts
for a site, because we don't need to use them within the app. They
are needed for configuring the CDN, but we can generate them for
that purpose from the real hostnames.
Include aka hostnames in the hosts API

We need to configure the CDN to handle these hostnames in the same
way as their real hostnames, so they should be included in the data
we expose for that purpose.
Add response info and total into hosts API response

This is the minimum needed to make the response format consistent with
Whitehall's APIs, not including pagination which we don't need at the
moment.
Always return a JSON response for hosts index
Test hosts presenter in hosts presenter tests

The main logic for generating the response was moved from the
hosts controller into the hosts presenter, so the tests for it
should also be moved.
HostPresenter doesn't need to inherit from Struct
Tidy up and simplify specs

- remove unneeded host creation in hosts_controller_spec
- simplify setup with let! in hosts_presenter_spec
- simpler use of subject in host_spec and presenter specs
Clarify Host.aka_hostname and add spec for www2
Rename 'aka' option to 'use_aka_hostname'
Merge pull request #65 from alphagov/managed_by_transition

Add a 'managed_by_transition' flag to Site, and use it for a hosts API