Serve tracker pixel from Tangent's infrastructure

This is to enable some coarse metrics of who is using Oscar.  We just
look at the referer header to get a rough idea of which sites are using
Oscar.

This is an important measure for Tangent, Oscar's sponsor.  It's
documented in the release notes and we'll make sure everyone knows when
Oscar is released.  It's easy to opt out of.

Fixes #656
Adjust migration to handle data changing part specially

I can't recreate the original issue but, according to a comment by
@vdboor, this change should ensure that the date migration does not
conflict with the schema migration.

Going forwards, I will ensure migrations don't mix data and schema
changes but at this point, it seems better to adjust this migration
rather than renumber (which causes problems for anyone who has already
applied this migration).

Fixes #977 (hopefully)
Merge remote-tracking branch 'upstream/master'