Add the base user flow for adding related content

This follows a similar user journey as the equivalent artefact admin
view in panopticon.

Here we're just making sure that users can click on a link for a given
country and add a single related item with handling any of the API
integration.
Bump plek to 1.2.0

This is to pick up the new environment style.
Add a default content api bearer token when in development
Bump gds-api-adapters to 5.3.0

This is to pick up the new Panopticon.put_artefact and
Panopticon.artefact_for_slug style.
Add gds-api-adapters Panopticon test helpers

These will be used to help stub out requests.
Remove the btn-group wrapper div

It doesn't provide the automatic styling through bootstrap as
expected, so remove it as it's cruft.
Allow addition of related content

For a single travel advice page (country), allow adding a single
related piece of content from the list of Artefacts held inside of
Panopticon.

This just tests the integration layer up to the point that an HTTP
request is made to Panopticon's API. We won't actually know about state
changes from that point; only whether the request was successful or not.
Move panopticon artefact requests into 'before' test step
Provide extra select field for additional related content

When some related content exists for an artefact, show an
extra (un-selected) drop-down to set additional content with.
Provide a way of adding additional related content

When we already have related content present, provide a way of adding
even more through a simple "Add" button.
Provide an easy way of removing specific pieces of related content
Nest tests to denote that they're related to when a draft is present
When no Artefacts exist for a Country, redirect user back to list page

In the case that a draft is yet to be created for a given country,
redirect the user back to the Country list page with a relevant alert.
404 and return

Need the return to invoke the 404 response.
Move related items tests into their own spec file

There are now a number of tests for this feature, so it's probably
best to separate this out into another file.
Use the CONTENT_API_CREDENTIALS

There aren't any different credentials for app endpoint. Rather, it's
using an app-wide GDS API key.
404 and return
Provide some controller specs for countries#edit

Make sure some of the basic interactions we do work as expected
against the controller.

Most of these just check the response given that certain bits of data
are available.
Correctly provide a 404 when a country isn't found
If we're going to use LOTR references, use actual nations

"Middle Earth" is the fictional universe setting of LOTR. Properly,
Middle-earth is the central continent of the imagined world, not a
name of the entire world.

These are two actual nations from LOTR.