Use the default RTD theme. Fixes #1485

[ci skip]
Setup the RTD theme.

[ci skip]
Test overriding the context

[ci skip]
Add the Previous and Next buttons

[ci skip]
Remove html_context (does not work on RTD)

[ci skip]
Merge branch 'master' of github.com:ckan/ckan into 1485-rtd-theme
Docs theme footer fixes

Wrap it in a <footer> (the theme styles this, e.g. deemphasised text
color), add an <hr> like the theme's default footer has, add back in the
credit line for the theme, tidy up the code formatting.
Merge branch '1485-rtd-theme'
Fix a Sphinx rendering error