Make schema_filter the default view of schemas

[slinkp]

I've pretty consistently heard that the schema detail page (with stats, etc) is less useful default view than the schema filter page (with some news on it).
Eg.  http://demo.openblockproject.org/photos/filter/ is more useful than  http://demo.openblockproject.org/photos/.

Should make  http://demo.openblockproject.org/photos/ the filter view,
and add a new url  http://demo.openblockproject.org/photos/detail or some such.
And redirect .../filter to the new URL.

[Paul Winkler <slinkp@…>]

(In [b8f5bd564db1d610d5d6f223ea319d1ffd807c3a]) - Change URL of Schema to be the schema_filter() view; there's now a /detail view for schema_detail, closes #272.

• Add FeaturedTag? model for marking some NeighborNews? tags as special, eg for use in navigation.
• Fix error on FilterChain?.add(key, lookup) when key isn't a schemafield.
• Add comments re. Lookup.slug vs. Lookup.code
• Add context processor for getting FeaturedTags? whenever needed in templates.

[slinkp]

This ticket has been referenced in ticket #210:

Effectively fixed by doing #272