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.