Fix broken offer report generation

Was assuming model instance, but got a dict.
Support other slugifier function
Add doc on handling migrations when upgrading

Fixes #608
Add product carousel to offer list page

This commmit moves the loading of a product queryset for an offer from
the offer view to the offer model.  This allows it to be used in the
`list.html` template for offers.

Part of #596 - still needs some FED love from @informa though.
Merge pull request #532 into releases/0.5
Merge pull request #580 into releases/0.5
Add missing settings to docs

Plus some general reorganisation.
Fix a few sphinx warnings
Rework the docs index page
Adjust coverage to only include oscar package
Bump version to 0.5
Fix mapping quantities to selected lines
Add several small tweaks to accounts views

Marks two forgotten strings for translation, removes redundant code from
templates and fixes one typo.
Add release notes for 0.5
Added a few screenshots for the 0.5 release note
Update message files
Rename target for compiling messages
Use default filter to avoid None in templates
Adjust settings docs for slugify function
Merge pull request #622 into releases/0.5