Make mainstream links agnostic to their owner

Mainstream links are going to need to be able to be added from world
wide locations. This lets us create mainstream links for other content
types by just adding a new join table.
Add mainstream links to world locations
Allow modification of mainstream links in admin
Display mainstream links on world location page
Revert removal of title, summary and body from edition

Even though we don't have any code relying on the these fields being present,
we think that ActiveRecord is automatically including the field names in the
queries it generates. As such, the removal of these fields still causes a
problem with deployment as old processes will expect the fields to exist.

This commit amends the migration[1] so that it'll be a no-op when run on
production (it's already run everywhere else up to and including staging). It
also adds a migration to re-add the title, summary and body fields on all
machines that have already run the original migration. This is almost
identical to the migration we used last time we were in the mess[2].

[1] e01713decd5846580b6220af62a25f2c7ee2083a
[2] 8b26d249637255f71b0f633d2920c5b2273d116c
Split out Organisation#show into #documents

This should speed up the organisation pages dramatically, as they no
longer load documents on #show.

Additionally we're now paginating the list of organisation editions, to
make the featuring experience faster.

The navigation between tabs should remain the same, as I've co-opted the
tab interface to provide a proper URL instead of a fragment identifier.

https://www.pivotaltracker.com/story/show/44486835
Merge pull request #240 from alphagov/mainstream-links

Add mainstream links to world location pages
Merge pull request #242 from alphagov/split-out-organisation-documents

Split out Organisation#show into #documents

https://www.pivotaltracker.com/story/show/44486835
Merge pull request #243 from alphagov/revert-removal-of-title-and-summary-and-body-fields-from-editions

Revert removal of title, summary and body from edition
Fix the frontend js now the locaitons field has been renamed

https://www.pivotaltracker.com/story/show/44710407
Update locales and fix cucumber test

Cucumber test looked for old class name. Locale was needed for a
different test.
Use correct documents/org path to fix features

Also update template where this was hard coded.