Disable Version Sources on a Per-Feedstock Basis

[ytausch]

We should add a configuration option for conda-forge.yml that allows disabling some version sources for a particular feedstock.

Why do we need this?

1. Feedstocks oftentimes rely on the hash stability of GitHub-generated tarballs, however, they have proven to not being stable despite GitHub promising to announce any changes to tarball stability within a six-months notice. (Or has there been a announcement I am not aware of?)
2. This is why we want to use custom stable source archives provided by the repository maintainers.
3. However, there are instances like LLVM where the repository maintainers upload their custom source archives only some time after the release has been created on GitHub.
4. The bot then detects the new version from the GitHub RSS feed, but any update attempts fail because the source archive is not available yet. This has been observed here.

Our preferred solution is an option to disable the RSS version source (or any other source, of course) for instances like this.

[ytausch]

Oops, this seems to exist already: bot.version_updates.sources