Add JSON index page.

My go at fixing [bug 962732](https://bugzilla.mozilla.org/show_bug.cgi?id=962732).
Adds a `/json-snippets/` page that lists all the JSON snippets with the relevant info (name, text, url and icon). This is basically taking the snippets page and reusing it with some minor modifications and looks like this:
![screen shot 2014-11-05 at 14 33 08](https://cloud.githubusercontent.com/assets/318208/4927598/073a8eac-653d-11e4-8880-a5518f3b0734.png).

Here's an idea. This view shares most the code with `index`. We can add a parameter to `index` (e.g. `view`) and then do something like this:

```
def index(request, view):
  if view == 'jsonsnippets':
    snippets = JSONSnippet.objects.filter(disabled=False)
    snippetsfilter = JSONSnippetFilter(request.GET, snippets)
    template = 'base/json.html'
  else:
    snippets = Snippet.objects.filter(disabled=False) 
    snippetsfilter = SnippetFilter(request.GET, snippets)
    template = 'base/index.html'
```

For this to work you need to set `view` in urls.py like this:
```url(r'^json-snippets/', views.json_snippets, {'view': 'jsonsnippets'}, name='base.json'),```