Make editions have many document_series

Add a join table for edition_document_series
Add model and modify tests & associations

Importer changes: document_series is no longer an attribute
should use the followin in csv's instead document_series_1,
document_series_2, document_series_3, document_series_4

Remove document_series_id from editions
Move the data into EditionDocumentSeries

Add list_of_links_to_document_series helper

Added an extra test and ensured we pass the document_series_ids
Merge pull request #147 from alphagov/edition-has-many-document-series

Change Editions to have many document_series
Add first_published_at field to detailed guides
Fix test: missing edition_class variable
Merge pull request #150 from alphagov/add-first-published-to-guidance

Add `first_published_at` field to detailed guides
ci build failing revert assert change
fix tests: document_series is a has_many now, should have been merged
fix tests: use assert_equal
Fix cucumber fixtures

thought this was already in there, must have lost a commit when
rebasing