[#884 #905] Styling on registration complate page fixed
[#823] Updated 403, 404 and 500 pages
[#823] Approve user in user management table

The user management table now has the following logic:
- A green check is shown if an employment is not approved yet
- A red cross is shown if an employment is approved
- Nothing is shown if the (logged in) user is not connected to the organisation in the employment

Also, the user management link and page (and API calls) are only available to superusers, staff members, editors and organisation admins.
[#823] Missing trailing enter
[#905] Updated signal to send email when employment is created
[#884] Updated user approval process

It should be possible to request to join an organisation again, and emails wil be sent out when a user does so.
In addition, the user management page is updated;
- The logged in user is not shown in the table
- Only users of approved organisations (of the logged in user) are shown
- The permissions column has been removed, since we need to re-think permissions (read: implement object-level permissions)

Also, on the my details page, the organisation list shows when a user is not approved for an organisation.
[#884] Updated 'approve' REST view and Modal title
[#884] Gulp fix for *.scss files
Merge pull request #907 from akvo/feature/884_v3_registration

[#823 #884 #905] Approve users and updated registration process