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.
Merge remote-tracking branch 'origin/master'