Split off some content previously placed into base back into index.html.
Move content previously localised in accounts_home to a new content_base
for use in the non-app version of the site.
We wrote up some copy and styled it according to taste.
In our new signup form, we no longer collect a short name and instead
extrapolate from the email.
(imported from commit cd734b13a4bda7f304129379524c4cc96b77b24b)
