Use the default RTD theme. Fixes #1485

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Setup the RTD theme.

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Test overriding the context

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Add the Previous and Next buttons

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Remove html_context (does not work on RTD)

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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'