Regsys admin page witth view/edit of registrations

[james64]

Add so called regsys-admin page accessible to velehrad-admin. It lists all registrations and is able to edit them. This replaces django-admin page completely.

This is not strictly necessary for regsys to be used in production.

Lack of such admin page was only reason for email sending implemented the way it is specified in #1. Once we can handle edit, cancel and other status changing operations on registration through regsys-admin we can send email immediately when reg is cancelled for example. This was not possible when status was cancelled by manually editing it in django-admin.