Use waffle Switch homepage_2015

I  reverted the original view to the version in the current master, then implemented a new view and associated helper functions to use a waffle Switch, as we discussed earlier.

To test this locally, when you first load the homepage you should see exactly what's in current master.
To see the new homepage:
`./manage.py shell_plus`
```python
Switch.objects.create(name='homepage_2015', active=True)
```
Then restart your dev server to flush LocMemCache (or flush memcached if you're using it), and you should see the new homepage.
My understanding is that we would continue to serve the old home page for locales without this tag-- this logic was in a template already, I just moved it to the view. However, I just realized this PR doesn't include removing the now unused template that implemented this logic-- will update shortly.