Link numbas extensions to git for version control

Also, when adding an extension to a question the editor could allow the user to choose "latest" or some git tag to fix the version of the extension in use.

Start by locating the editor flow for adding an extension to a question and inspect how the extension version is currently stored. Determine how the UI should represent “latest” and a git tag; done means an extension can be added with either choice and the selected version is retained.

Written by the indexing model from the issue text.