support functions for org-specific base templates per domain

We want to be able to support organization-branded versions of
the DDR.  Partners will want to display their collections (and
only theirs) and customize the UI.

settings.ORG_DOMAINS is used to list domains that belong to
specific organizations.  If the request domain matches an
organization domain the org keyword is added to sitewide context.
If org-specific base template exists it also is added.
mv functions from context_processors to ui/__init__
templates extend BASE_TEMPLATE from sitewide context
"org"->"partner" for less confusion; domains_org only returns org

- Outside of the domain_org and base template functions, refer
  to "partner" instead of "org" or "organization", since these
  are already used.
- Calling functions use the org/partner from domain_org but not
  the domain, so only return the org/partner.
__repr__ methods so the models look nice in tracebacks
collection/entity/file views filter by partner, if present

If ui.domain_org finds a partner,
- Collections list pages only list that partner's collections
- list and detail views raise Http404 if a different partner
  is requested.

NOTE: facets and search results are not yet filtered!
sample base templates for hmwf and janm