Refactor login template to extend base.

While experimenting with extending DRF, I found that the login page
1) had no title, and 2) duplicated <head> info from base.html.

This change adds a new {% block body %} to the base.html template
which allows override of the entire html <body>. login_base.html
has its duplicated head info stripped, and now extends base.html
to share common html <head> templating.

As part of this change, pretify.css is unnecessarily added to
login_base.html.  If this is deemed a problem, it will be easy to
block that css out, and have login_base.html override the block.

Ideally, I would have liked to create a new api_base.html that extends
base.html, move the api specific logic into that template, and leave
base.html content agnostic, to truely be a unifying base for all DRF
pages.  But this change would break current apps that override
api.html and expect base.html to be the immediate super template. :/

This change is benificial because it:
 - removes duplication of header declarations (mostly css includes)
 - adds a html title to the login page
 - standardizes html header info across all DRF pages

Docs are updated to reflect the new structure.
Merge pull request #1641 from javins/login-title

Refactor login template to extend base.