Enable user organisation editing

Turns on editing of a user's organisation with a feature flag and adds a script which can be used to import organisation membership for all users in whitehall. This will be followed up by a change to whitehall which disables editing a user's organisation at which point signonotron will be the authoritative source for user organisations.

Inside Gov ticket: https://www.pivotaltracker.com/story/show/60396766

The corresponding pull request for whitehall is https://github.com/alphagov/whitehall/pull/1099. This pull request on signonotron should be merged and deployed and the `import_user_organisation_membership_from_whitehall` script should be run before the whitehall pull request is merged.