Allow loading logic that doesn't depend on ThebeButtonNode

[choldgraf]

Description / Summary

Right now sphinx-thebe will only load if it detects ThebeButtonNode on a page. However, if a theme is using sphinx-thebe to load thebe, but not the ThebeButtonNode, then thebe won't load for them.
We should find some way around this and also determine how common of a use-case this is.

Value / benefit

This is useful for anyone that wants to piggy-back off of this extension but without using the directives provided by this theme. A good examples of this is the Sphinx Book Theme, which manually activates thebe:

https://github.com/executablebooks/sphinx-book-theme/blob/9a2da30342ace86978ddbd345eb69f5b4479b31c/sphinx_book_theme/topbar/launchbuttons.html#L29

and this logic is triggered here:

https://github.com/executablebooks/sphinx-book-theme/blob/e05def5b9a4fc777f702553a0bcaf7939440dbd3/sphinx_book_theme/launch.py#L118

Implementation details

A couple ideas:

• Check for an in-page metadata like thebe: true
• Check for an html context variable like context['use_thebe']
• Check for some other kind of in-page content (e.g. special case the class thebe-launch-button?)
• Make it configurable to use any other kind of object in the doctree

Tasks to complete

• Decide what's the best way to support other kinds of nodes/objects for thebe loading
• Implement

[choldgraf]

cc @akhmerov - do you have any thoughts about how we could most-easily decide whether to load thebe in projects that aren't using the directive here?

[akhmerov]

I have refreshed my memory of how sphinx-thebe works, and indeed understand the problem now, with the selector being invisible as long as we only have the AST.

Would it make sense to change the sphinx-thebe configuration to take a node or a list of nodes instead, and add the class = 'thebe-container' attribute or something like that to those nodes?