Refactor Usersuite

[lukasjuhrich]

Following and slightly agreeing with the codeclimate report on usersuite.py, it might be good to resolve some of the mentioned issues.

For instance, a lot of repetition could be cut down by introducing a context manager in a manner like that:

@bp.route('/route')
def route():
    with process_form(LoginForm):
        # the stuff that usually comes after `validate_on_submit`
    return render_template('route.html', foo="bar")