Event duration

[sportfloh]

This pr adds the ability to add a end date time to an event.

Defaults: Tech-Event: 14 - 17 h Chaostreff: 20 - 23 h Freies Hacken: 14 - 17 h 3DD: 20 - 22 h

Things I've done:

• Added the end field to the event model
• Adapted the templates to check for the new field and use it if it exists
• Manually added end date time to already scheduled events

[justinotherguy]

lgtm - thx! :)