Add event templates

[PurkkaKoodari]

In addition to event copying (#54), it would be a good idea to have boilerplate templates for events, like "sitsit", "culture event", etc.

In practice, this would be easiest to implement by:

• Adding a "template" flag to events (maybe replacing the "draft" flag with a "type" field)
• Showing templates only in a separate list
• Reusing the editor, possibly with a "convert to template" button
• Adding a "create from template" option when creating events, implemented in the background by just a "copy event" action