Use our own jinja2 template loader

[yuvipanda]

• Our template loader setup was a hack, and that hack seems to
  have stopped working.
• We just use our own loader instead. This means we lose the base
  translation functionality of upstream render_template - but we
  were not using it at all anyway, so 'tis alright.
• Looks like the path for the stylesheet we were 'borrowing'
  from classic notebook changed location, so update to use the
  new location. This will stop existing after notebook v7,
  and we will need to use our own.

Fixes #235

[yuvipanda]

@manics this is an alternative to #240. We need to do #240 as well regardless, but I don't understand the implications enough.

/cc @Zsailer who helped point this out.

[yuvipanda]

@betolink you might be interested in this too.

[choldgraf]

This feels like a cleaner implementation, and since it fixes a nasty bug I think this is a great improvement! +1 from me unless somebody uncovers any other technical issues

[yuvipanda]

I think #240 is also needed, but @Zsailer told me more is needed to turn it into a proper jupyter-server application. I don't know enough about what more needs to be done there though.

[Zsailer]

Can someone here make it to one our Jupyter Server Contributor Hours? 😃

It would be really fun to try and port this extension in real-time. Then, we can process record, document, and share this in the server docs. I'm happy to lead the development and we can pair program as a team during the call.

[choldgraf]

Just to be clear @Zsailer - you're talking about the resolution of this issue, right?

• Depend on jupyter-server only, compatibility with jupyter server >= 2, notebook < 7 #240

Agree that would be great :-) but I'm still +1 on merging this PR in the meantime!

[Zsailer]

Ah, yes—I was referring to #240. Sorry for the confusion! 😃

[manics]

Thanks!

[yuvipanda]

Thanks a lot, @manics! Do you think we can do a release?

[manics]

I think so, if we don't need anything from #240