[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)
Sorry for the late answer, I've been quite busy :)

Looking at this again I've got to say that I still think that this should be `event_id`. In my opinion it's not equivalent of calling a `description_string` since this variable is the id of a event and not the event itself and calling it `event` would be wrong. The equivalent would be calling it `event_int` which would be not good either. To further illustrate my point I think that we'll run into some problems with naming variables when in situations like such:

```python
def edit_event(request, event):  # Event here is the id of the event not the actual event.
    actual_event = Event.objects.get(id=event)
    ...
```

To me this feels very unreadable and I would strongly prefer this form:

```python
def edit_event(request, event_id):
    event = Event.objects.get(id=event_id)
    ...
```