Copy event

[ZuzkaU]

It should be possible to copy an event, keeping all the values except the date. The problems with the templates are:

• events evolve, and a change made in the event description is not always updated in the template
• even when using a template, it is needed to fill a lot of values manually
• templates should be more general, but specific events can vary (e.g. same event can have different location each time, or some features which we only want to use sometimes, such as description fitted to Christmas edition)

(also solves #576)