Jujutsu/git integration

The editor now has a button to commit changes to a Jujutsu repository,
which is automatically created if it's not present.

A git remote is automatically set up using the URL template in
settings.py. I use this with Forgejo's create-on-push feature to
automatically create repositories on my Forgejo instance.

.gitignore

@@ -5,3 +5,4 @@ public/
 secret_key.txt
 think_data/
 Makefile
+thinkserver/settings.py