Make it optional to redirect to the parent

Fixes #1420
Rename template references to variants

Where I previously thought that renaming HTML/CSS is more hassle than
it's worth, @mbertheau convinced me that to update a codebase, a general
search-and-replace is the best approach already and will catch any
HTML/CSS changes.

I also missed to deprecate the min_variant_price properties.
Ignore capitalisation of local part of email address

Most email servers don't respect capitalisation, and many users don't
know about it. So Oscar now does what the rest of the world does, and
ignores the capitalisation when looking up an email address.

Fixes #1179.
Correct import path

A gremlin spawned in 860fab7ba770dbaf12564ec8434b65323e616ae1. This was
bringing down the sandbox site.
Add robots.txt files to latest/demo sites
Ensure only signed-in users can vote

Not sure how this bug has survived this long - it was well masked as the
template will hide voting buttons for unauthenticated users.
Correct proxy IP in nginx config