gitweb: Disable snapshot feature

https://discuss.freedombox.org/t/gitweb-snapshot-requests-cause-low-memory/2502

The snapshot feature is enabled by default, which does the following:

Enable and configure the "snapshot" action, which allows user to download a compressed archive of any tree or commit, as produced by git-archive[1] and possibly additionally compressed. This can potentially generate high traffic if you have large project.

If there is some interest in having gitweb snapshots, we can have an option to enable it. It can also be enabled per repository.