T423 link to articlesplus

This pull request introduces a new settings variable `EXPLORE_TYPE` which can be set to either `surveyor` or `summon` which controls what launchpad links to when exploring authors, subjects and series titles. The templates use three new templatetags: `explore_author`, `explore_subject` and `explore_series` which generate a URL based on a supplied string, where the string is an author, a subject or series title respectively. There are some tests in `ui.tests.explore_test` that exercises it, and shows you how to use it.

When we truly move over to using Summon, and abandon Surveyor for these links we will want to remove the `is_roman` logic in the `item.html` template, which restricts linking out to iso-8859-1 text. This is a limitation of Surveyor but not Summon. It proved to be a bit unwieldy to bundle this logic in the explore_* templatetags.

Fixes #423 
Fixes #412