Make the thebe package URL configurable #24

Description

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

and loaded here

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.