Make the thebe package URL configurable

[akhmerov]

Description

Right now the thebe package is hard-coded in the codebase:

sphinx-thebe/sphinx_thebe/__init__.py

Lines 12 to 16 in c036ac4

	logger = logging.getLogger(__name__)
	THEBE_VERSION = "0.5.1"

and loaded here

sphinx-thebe/sphinx_thebe/__init__.py

Lines 66 to 71 in c036ac4

	# Add core libraries
	opts = {"async": "async"}
	app.add_js_file(
	filename=f"https://unpkg.com/thebe@{THEBE_VERSION}/lib/index.js", **opts
	)

This should be configurable for the documentation author.

Benefit

1. Thebe intends to introduce breaking changes (see e.g. Finish the rename + migration jupyter-book/thebe#230)
2. Now it's published under a different name (thebe instead of thebelab)
3. Some applications may prefer to ship their own version of the js library (e.g. for working offline, etc).
4. Some smaller/stale packages won't rebuild often. This means their deployed documentation will break.

Implementation

This change would entail adding a new configuration value like thebe_version. It could either be just a version string, or it could be a link to the whole thebe bundle if it's better to manually specify this.

[choldgraf]

Maybe a config called thebe_url ?

[choldgraf]

@akhmerov just FYI the version is now pinned, so I'm scoping this issue just to configurability

https://github.com/executablebooks/sphinx-thebe/blob/master/sphinx_thebe/__init__.py#L134