[#1294] Update docs theme to latest version
[#1294] Fix logo on docs header bar

The logo needs to be in a _static folder because the sphinx-theme-okfn
hardcodes it there:

https://github.com/okfn/sphinx-theme-okfn/blob/master/layout.html#L18
Merge branch 'master' of github.com:okfn/ckan into 1294-update-docs-theme
[#1294] Show table of contents in docs sidebar on front page
Merge branch 'master' of github.com:okfn/ckan into 1294-update-docs-theme
Merge branch '1294-update-docs-theme'