Dataset collection - update edit page

[langphil]

As part of the current development milestone to make octopub into more of a pre-publishing tool (and not merely a publishing tool) we need to allow the users to edit a collection in two different contexts:

• while the collection has not yet been published (anything can be edited)
• once the collection has been published (files can be added and descriptions can be amended, and the result can be re-published.)

Per elaboration workshop on Wed 9th May 2018, we have established that this feature update requires the following:

• add menu action for 'edit' on collection dashboard
• confirm upate of collection details still works - name not editable if in post published state
• DB migration - published status for datafile
• DB migration - third status on collection - implement page logic
• confirm can update file descriptions for existing published files
• remove 'accept csv'
• prevent reupload for published files - replace with read only details / links to s3/github
• Cancel adding futher files (bugfix)

[caiwilliamson]

#828