Fixes #3511: Check Git user before committing, set dummy user if necessary

[danepowell]

Fixes #3511

Changes proposed

• Better document git.user.name and git.user.email config properties in build.yml (these already existed before as an undocumented feature for deploys)
• Add a validation function (validateGitConfig) that ensures that the Git user is set correctly either via the system or via the above config properties
• Use this validation function prior to deploys and new project creation (the only two places we do Git commits)
• If git.user.name and git.user.email are set, add an --author argument to Git commits during deploys and new project creation.
• Add an empty "precommit" during project creation to make @TravisCarden happy 😄

I haven't fully tested this yet, it very well might not work just yet. There's probably some opportunity to refactor the construction of the commit command. But I'd still appreciate any feedback.

@ba66e77 this is modifying a feature that you originally requested in #1323 , on the off chance that that is something precious to you. I think it should operate basically the same, except that we'll set the Git user on each commit rather than once for the whole repo.

[TravisCarden]

I haven't tested it, either, but the design seems good, and the implementation looks reasonable.

[danepowell]

I refactored the Git commands into their own class, so we can set the author consistently for commits.