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.
Any reason for the `+ 302` here? Seems to scroll into the middle of the promos - is that by design?