Enhance custom site backup to GIT

[jgstew]

There is existing functionality for export_all_sites that can be used to "backup" custom content on a regular basis to GIT.

• https://github.com/jgstew/besapi/blob/master/examples/export_all_sites.py
• https://github.com/jgstew/besapi/blob/e0e5058fcbe337632f76777ec1a020476e621dc9/src/besapi/besapi.py#L744

It should be enhanced to only export contents if the last_modified time is newer in the origin site than in the already exported file.

The goal is to allow one way "backup" of custom site(s) to GIT, then allow one way ingestion of the same site from GIT.

Related:

• https://stackoverflow.com/questions/4565700/how-to-specify-the-private-ssh-key-to-use-when-executing-shell-command-on-git