✨ IMPROVE: Conditionally load on pages

[choldgraf]

This conditionally loads thebe so that the JS is only loaded on pages that have a thebe button.

supercedes #14

Also pins our Sphinx lower bound to 3.5 since that is when conditional loading was introduced.

[akhmerov]

What happens if the button is provided by the theme?

[choldgraf]

Right now it will purely look for ThebeButtonNode and if that doesn't exist, it won't load thebe. I guess the assumption is that if the theme is hard-coding a button then they could also manually load the JS itself? What do you think is a good solution there (other than loading it on every single page regardless of whether there's a button on it)?

[akhmerov]

Perhaps checking if thebe is configured and there are cells to execute? This seems to be agnostic with respect to the execution method.

[choldgraf]

@akhmerov I opened up this one to track this: #34

I think the main question is "are there cells to execute". e.g., how do you define what that means? (wanna take conversation over to #34?)