Add fields `external_registration` and `type` to the events resource

Created by: temparus

The requirement emerged to have events organized solely by companies (with their own registration tool) listed on a separate page on the amiv website.

In order to be able to distinguish those different event types, a new field type is introduced which adds three different types of events:

Type	Description
internal	Events organized by amiv. These are the events as we have them so far in our system.
external	Events organized by companies which are listed on a completely separate page on the amiv website.
announcement	Special event type for future use to be able to show announcements on the amiv website and other infoscreens without showing them on the events page.

In addition we also introduce the new field external_registration which accepts an URL pointing to an external registration website. This field cannot be used together with our internal registration tool. The primary idea behind this field was to have the registration link for the external company events in a structured manner. But we do not want to limit this feature solely to external events, so it can also be used by other event types (e.g. Coding Weekend where the registration is hosted on the VIS website). This new field accepts HTTP and HTTPS links only.

Important Note

As we introduce an new required field (type), we have to migrate the existing events to have that field. As we do not have any structure for automatic database migrations, we should run a data migration script when this feature is released to master and deployed on production.