[533] Themes prototype

WARNING: The styling sucks.

Requires theme-primary adding to facet list on configuration

These changes add several new features:

 - Tag facets on the search page have been moved down and themes have been inserted near the top.
 - There is a new themes page at /data/themes that shows all the themes and has some space for some text etc.
 - A theme homepage exists at /data/themes/theme such as /data/themes/health which shows several popular, and new datasets in that theme as well as boxes for the drupal information.
 - The search box on a theme homepage just applies a facet and searchs on /data/search
 - A tool for 'guessing' what theme a particular dataset should have based on the publisher.  It isn't perfect and still leaves a lot not set, but at least shows what the theme stuff could look like.

    python ckanext/dgu/bin/theme-guesser.py n

Replace the final n with y when ready to run and commit changes.

Needs:
  - Per theme dataset icons (to differentiate them).
  - Styling properly.
  - An API from drupal to get apps/ideas for a theme.
[384] Store theme-secondary as a multivalued type
[384] Added a themes search facet. Updates the schema and plugin implementation.
[384] UI updated. All themes can be selected as secondary. ('location' was unavailable')
[384] Code to render themes on datasets in search view.
[384] Merged Ross' theme branch
Themes visible on datasets (needs work). Can drill down by Theme or by Primary-Theme. UI incomplete.
[384] JS clickbox to search by primary theme. Or not.
[384] Cleaning up theme rendering
[384] Stock photography images for use on theme page.
[384] Redrafted theme master page.
[384] Shuffle theme page content
[384] Do not expect pkg_dict to contain theme-primary/secondary
Merge branch 'master' into 384-themes
[384] One-line CSS fix for Chrome on Ubuntu
Merging master, re-running grunt
[229] Add schema.xml improvements to v1.5 schema.
[384] Themes added to v1.5 solr schema.
[384] Don't allow duplicate themes into the search index (eg. where Primary theme is also Secondary)
[384] refine_packages command handles secondary themes much more carefully & cleanly.