Delete legacy data

This data was imported long ago and isn't required anymore.
Get rid of Travis config

Now that Jenkins does branch builds, we don't use Travis anymore.
Delete old migrations

Old migrations are not needed any more. This gets rid of all those older
than one month.
Delete old one-off scripts

These have all passed their sell by date and are no longer used. The
cost of maintaining/updating them when stuff changes far outweighs the
benefit of having them hang around just in case we need them again one
day.
Get rid of redundant seed data

None of this is used anymore.
Merge pull request #1560 from alphagov/delete-cruft

Delete cruft
Bump Plek gem

The new version gives us Plek.new.website_uri
Use shorthand for conditional assignment

We can rely on these ENV vars being nil rather than blank as they are
highly unlikely to be set with an empty value
Whitehall.asset_host goes via Plek

We shouldn't need to access ENV vars from within the app. Plek should be
giving us exactly what we need.
Change default static asset serving in dev mode

Up to now, whitehall has defaulted to serving assets from preview in
development mode. This is the opposite of how all the other apps in our
stack works. This reverses that so that whitehall expects assets to be
served from wherever Plek says they will be, but also allows the static
server to be overridden directly with an ENV var (STATIC_DEV) if
required. For convenience for those developers who are running whitehall
outside of the VM, startup.sh has been updated to set this ENV var so
assets are served from preview.
Remove unused initializer

This initializer is no longer used as staging gets the right asset host
as of alphagov-deployment commit 12c3eefb6777881fea48c24638b04db2f6f3c07d
There should only be one Whitehall#admin_host

Plek gives us the correct host based on the current environement.
Whitehall.public_host derived from Plek

Up to now, we've set Whitehall.public_host in an initializer that is
overridden per-environment during deploy. Plek is the source of all
truth so we do not need to do this.
Remove more hardcoded URLs

In most cases, the default Whitehall.url_maker will give us what we need
to generate both public and admin urls. Some tests required slight
adjustment to reflect the slight difference in the newly implicit
protocol (derived from the environment) and the previously explicit one
hardcoded into the source.
DRY up url generation of atom feeds
Get rid of unused methods
Use consistent domain for test env
Don't special case protocol scheme

Once again, Plek tells us the truth.
Remove redundant code

This is no longer relevant
Remove more redundant code

Direct route registration is no longer performed - route registration
happens during panopticon registration now.