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
Add another contextual marker for translations

This should have been part of d3ddfa5. Also see f9436dc.

Closes #1421.
Rename base product to parent product in BasketForm

This does not change any functionality, just updates to the adopted
naming scheme.
Add product structure to sandbox fixtures

The T-Shirts were shown as not available; this commit fixes it.
Remove product overview section in product editing dashboard

It is a nice thought, but an uncommon pattern. It also does not add any
information over the 'View on site' public facing site. So let's KISS.
Reorder and rename product dashboard sections

This rearrangement was suggested by Tangent's UX team to try to model a
flow that's closer to how the mental process of adding a product goes.
Hide stock-related fields from display if not tracking stock

All three fields are irrelevant when not tracking stock, so hiding them
avoids confusion. Unfortunately, we need to inspect the product class to
decide whether to hide them or not, as we don't have a form to inspect
when rendering the table header.
Add semi-helpful pointer on how to create child products

It's currently a pain to create child products and will be addressed as
part of #675. We can nonetheless at least try to be helpful and explain
how to get things working till then.
Hide attributes section if product class doesn't define any

If the product class doesn't contain any attributes, the attributes
section in the product dashboard is empty and only serves to confuse.
Use an AutoSlug field with offer ranges

This prevents an IntegrityError from slugs clashing after renaming a
range to avoid the checked-for name clash.

Fixes #1424
Test that product deletion is handled gracefully in order lines

This was a concern raised by @sampaccoud, and it's best to have
certainty that deleting a product does not impact an order line more
than necessary.
Correct typo in checkout thank-you template
Correct translation mark-up for offer dashboard templates
Merge remote-tracking branch 'upstream/master'