Enable relating artefacts

https://www.pivotaltracker.com/story/show/69920830
merge and bump this first: https://github.com/alphagov/govuk_content_models/pull/163

re-enabling the relating artefacts feature, implemented
in a way that it can handle copious amount of relatable
artefacts.

as a first step, progressively enhancing, we're allowing
users to enter a comma-separated list of slugs related
to an artefact. this will be a fall-back method when the
page doesn't run javascript.

by the end of the story, we'll have a [select2](http://ivaynberg.github.io/select2/) text field
that will allow users to search for artefacts they want
to relate, when javascript is working on the page.

some tests and view code has been marked pending,
removed temporarily only till we start working on the
javascript version of this feature - https://github.com/alphagov/panopticon/commit/1cb9b740d3f2a7df9bb2b434a937a7d0a0f1e0de