Fix for Bug#1099253

Channel info shows up in search results. Added the slug to the test_searchy_by_channels case since I thought channels should point to a url if they show up in search results.
This is not efficient enough. It causes a ORM query for each event. We'll need to use a local "map" and use that to render here. 
Suppose we have a dict called `channels` where each key is the event then we can do this:
```
{% for channel in channels[event] %}
....
```
The way to get that can be to generate this dict in the view function and pass it into the render function. So currently we do something like this:

```python
return render(request, 'search/home.html', context)
```
but we need this to be:
```python
context['channels'] = event_to_channels_dict
return render(request, 'search/home.html', context)
```

What we want to achieve should technically only require 1 extra ORM query. We have a known list of events and thus we have their IDs so we should be able to use that to extract ALL channels that it touches. 

All it should ever require is 2 queries. One to extract the mappings of events to channels. And another to extract the channels as objects. 

So, to do this I wrote a new addition and added it to master. You're going to need this for this work and we're going to need it later when we possibly list the channels for each event on other browsing features. 

So, what you need to do is pull down the latest master, and rebase your branch against that and then you can do:
```python
from airmozilla.main.utils import get_event_channels
...
context['channels'] = get_event_channels(events_paged)
```
See [this commit](https://github.com/mozilla/airmozilla/commit/7060eeb2d21f892886f0e71bbfd59f2a5e453068)