[Fix bug 1097315] Add gallery select when working with events

Ok so here goes. This PR adds the ability to select a image from a gallery when creating a new event.

![screen shot 2014-11-14 at 13 13 46](https://cloud.githubusercontent.com/assets/318208/5053512/01a55c14-6c00-11e4-93a1-13eb29f0873c.png)
![screen shot 2014-11-14 at 13 13 53](https://cloud.githubusercontent.com/assets/318208/5053511/01a534b4-6c00-11e4-9e0f-27a6cc55008c.png)

Keep in mind that I don't have an eye for design!

[Bug 1097315](https://bugzilla.mozilla.org/show_bug.cgi?id=1097315)
I remember this being somewhat funky in the current infastructure so I made a little test with `pdb` to make sure that this is true and found out that event isn't always attached to the instance so replacing the `event_id` form property with `self.instance.event` is not possible.
```
 81         def clean(self):
  82             import pdb; pdb.set_trace()  # XXX BREAKPOINT
  83             cleaned_data = super(EventEditForm, self).clean()
  84  ->         event = Event.objects.get(id=cleaned_data.get('event_id'))
  85             placeholder_img = cleaned_data.get(
  86                 'placeholder_img') or event.placeholder_img
  87             picture = cleaned_data.get('picture') or event.picture
  88             if not placeholder_img and not picture:
  89                 raise forms.ValidationError('Events needs to have a picture')
  90             return cleaned_data
(Pdb++) self.instance.event
*** DoesNotExist:
(Pdb++) cleaned_data.get('event_id')
u'22'
(Pdb++)
```