Create both readonly and read-write users with setpassword command

Extended setpassword management command with two new features:

- Command now creates both readonly user and admin user
- The admin user now receives the `write_access` permission
Implemented login with two different users

The user can now choose in the login form whether he wants to login as
readonly user or as read/write user.
Fixed broken regular expressions in urls.py

The URL definitions in `tncapp/urls.py` contained some broken regular
expressions (missing trailing slash). This commit fixes these.
Check permissions in backend

Added `@permission_required` checks in all relevant views.

There is now a new test function in `tests/test_auth.py` that verifies
permissions on all the relevant URLs. If new create/update/delete URLs
are added, the test parameters should be adjusted accordingly.
Check permissions in frontend

Form fields are disabled using the new `{% input_visibility %}`
template tag.

Other blocks are checked using the following `if`-statement:

    {% if 'tncapp.write_access' in perms %}