DM-44461: Refresh notebooks

[fajpunk]

• Add a refreshing attribute to Businesses, with an endpoint and a MonkeyFlocker command to set it. Businesses can handle it however they want.
• NotebookRunner businesses handle this by recloning their notebook repo and forcing a new session.
• Add a GitHub webhook handler to set the refreshing attribute on NotebookRunner businesses for push events from matching repos.
• Update dependencies

I originally wanted to handle this by stopping and starting the flock via the flock manager, but I kept running into conflicts between multiple restart requests, and conflicts between restart requests and the business-as-usual starting and stopping of Jupyter labs and sessions during normal NotebookRunner flock executions. I couldn't work through all of these in the time that I gave myself, so I ditched that plan in favor of what you see here.

[athornton]

Typing error:

src/mobu/handlers/external.py:264: error: Missing return statement [return]

Usually before pushing a commit I like to do tox -e typing, tox -e lint, and tox -e py in that order (because that tends to be least-to-most-time-consuming-to-fix, in order).

[rra]

Overall, this looks great! I like how the refreshing works.

[rra]

Needs a scriv file for the change log and I noticed a couple of very minor nits, but otherwise looks great!