Implement homepage redesign

To see data on the homepage run the following cron jobs:

`./manage.py cron update_reps_ical`
`./manage.py cron update_openstandard`
`./manage.py cron update_tweets`

This PR needs to merge after the /contribute page, as some links need to be updated to point to pages not yet in master.

I still have some tidying up to do, but this should be close enough for code review to begin.
I've tested this a fair bit, and I think in this instance it is worth adding a class to the promo container on scroll. Page scrolling is a fair bit smoother in Chrome for instance, if the large hover promo transitions don't trigger on scroll.