Update style-sheet links to reflect those from jupyter-server

[possiblyMikeB]

At some point the jupyter-server project switched from including a combined style-sheet in their page.html template to including three distinct style-sheets.

This pull request changes the head block of the template page.html packaged with nbgitpuller to be consistent with the template of the same name from the jupyter-server package.

This change resolves #358

[agoose77]

@possiblyMikeB thank you for this PR!

@sgibson91 and I confirmed that this fixes #358.¹ Your fix pulls in the dependencies for styling from the upstream jupyter-server. As of the time of writing, Jupyter Server ships these files for all versions of jupyter-server supported by nbgitpuller.

This may change in the future, and we think that another PR down the road should look at bundling bootstrap (or dropping it entirely) into nbgitpuller.

For now, this LGTU (looks good to us!)

Footnotes

1. https://github.com/2i2c-org/infrastructure/issues/5059 ↩