- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

Also:
- change the default value of the "target_path" argument
- improve documentation of the "target_path" argument
- fix old documentation that says frontend and dashboard use different
  Bootstrap versions

`make css` has been renamed to `make assets`.

I struggled with migrations when upgrading from Oscar 2.0.4 to 2.1.
@solarissmoke kindly helped out, and I want to say thank you by adding
the bits I learned to the docs.

Where an initial value is required, this should be set on the form field
itself.

Fixes #3590.

Fixes #3517.

Co-authored-by: Basil Dubyk <samitnuk.work@gmail.com>
Co-authored-by: Alexander Gaevsky <sasha@sasha0.ru>
Co-authored-by: Joseph Wayodi <jwayodi@gmail.com>

Fixes #3258.

Non-exclusive offers that only allow combination with specific other offers

Also remove unused migrations test for MySQL.

These release notes were created in 6a03a0bc1f and 05b14af8af, but
somehow didn't make it into the master branch.

This is why http://docs.oscarcommerce.com/en/latest/releases/index.html
currently doesn't mention these releases.

Currently, if a valid basket line is marked for deletion, any other
invalid basket lines on the formset prevent it from being deleted.