Rename offer migration to ensure number uniqueness

This will cause a few problems for sites that have already applied this
migration, but I think it's better in the long-run to have the
migrations use a unique number.
Allow Sphinx==1.2
Document migrations in 0.6 release notes
Reset page number when building facet URLs

It doesn't make sense to carry it through to the constructed URLs.

Fixes #1114
Avoid rendering "None" for HTML meta-descriptions

As noted in #1113
Strip html tags from category and product meta descriptions.
Remove old checkout decorators

Don't think they are used anyway anymore.
Improve core documentation

- Expand the ref/apps docs. (Still lots to do here.)
- Split up the index page links for clarity.
Expand 0.6 release notes

- Include screenshots.
- Expand sections up to deprecation.
Use RTD theme locally
Use Napoleon Sphinx extension
Configure settings for docs using test function (not sandbox)

This avoids a pesky autodoc import issue that the debug toolbar creates when it
patches the URL conf.  It was preventing the EventHandler class from
being loaded via autodoc.  This fix solves that issue.
Improve core functionality docs

Now using Napoleon :)
Add release note for 0.5.3
Remove dead fields from phone number functionality
Document new model fields for release notes

Plus ensure the copyright notice is correct for the modified phone
number work.
Update deprecated features notes in release notes
Correct a few references in availability/prices doc
Update translation files from Transifex
Bump version to 0.6