Don't use site_return_path for bulk adding

It makes more sense to just return to the site mappings index after
creating multiple mappings, because the user is unlikely to have added
mappings which would then all be visible in the filtered and paginated
view. The main mappings index showing the total number of mappings will
probably be more helpful for the user to check that mappings were
correctly created if they want to.
Actually check message for unauthorized user

If we are redirecting to the site mappings index on both error and
success, we also need to check the message to be sure that the right
condition was reached.
Use shared examples for user-can't-edit tests
Pass return URL through bulk edit forms

This no longer stores a return URL per site in the session cookie,
which resolves the bug involving the session cookie growing too large
for nginx to accept the request. It also means that if a user is
working on a single site across multiple tabs, their histories will
not get confused.
Move return_url onto BulkEditor

Also add comments to case.
Include return_url hidden in mappings index table

This means that we don't need to use HTTP_REFERER.
Use path instead of full URL to return to
Don't set initial_return_path on controller
Only check that return_path looks like a path

We don't need this to be specifically a site mappings index path.
Also clarify the comment on this check.
Add a feature to test presence and use of return_url
Fix ||= for return_path
Split long post lines in mappings controller spec
Merge pull request #154 from alphagov/return_path_without_cookie

Don't store return URL for bulk editing on session