Warning messages in book template (extra_navbar, extra_footer, test notebook)

Describe the bug Defining extra_footer has no effect and there are several warning messages about extra_navbar, extra_footer, and a notebook file in the template book that is provided.

To Reproduce Steps to reproduce the behavior:

1. Create a new jupyter book by typing jupyter-book create testbook
2. Build the book with jupyter-book build testbook
3. View the build output to see the warnings.

Expected behavior I would expect not to see warning messages in the template, and also to be able to change the default footer by changing the configuration option for extra_footer.

See the output from building the template below.

$ jupyter-book clean testbook; jupyter-book build testbook
Running Sphinx v2.4.4
making output directory... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 4 source files that are out of date
updating environment: [new config] 4 added, 4 changed, 0 removed
checking for /private/tmp/testbook/references.bib in bibtex cache... not found
parsing bibtex file /private/tmp/testbook/references.bib... parsed 5 entries
Executing: notebooks100%] notebooks

looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... WARNING: unsupported theme option 'extra_navbar' given
WARNING: unsupported theme option 'extra_footer' given
done
writing output... [100%] notebooks
/private/tmp/testbook/notebooks.ipynb:10032: WARNING: unknown document: syntax
generating indices...  genindexdone
writing additional pages...  searchdone
copying images... [100%] _build/jupyter_execute/notebooks_2_0.png
copying static files... ... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 3 warnings.

Environment (please complete the following information):

• Python Version 3.8.2
• Package Version 0.7.0b4

[choldgraf]

gonna close this one and open up https://github.com/executablebooks/jupyter-book/issues/675 to track

[choldgraf]

Hmmm - so @MasterScrat first off could you upgrade to the latest beta? This was very recently added in.

@davewhipp could you try manually upgrading your sphinx book theme to see if this fixes things? (pip install -U sphinx-book-theme)