GitHub: allow optional commit message on generate

[derekperkins]

Rather than always committing with the Updated/Added simply-static-1-1680103088 message, it would be nice if there were an option to add a commit message, both for full site generation and single post. If GitHub is the selected deployment mechanism, have a text box where you could say something meaningful, like Yoast: changed sitemap settings, or Post: added internal link to pricing page. That would make the git history infinitely more usable.

Full site generation

Text box right above the generate button and below the dropdown

Single post publishing

Either above or below the button

[derekperkins]

It's possible that the same text box might be useful for other deployment mechanisms as a deployment name / description, but I'm not familiar with them.

[patrickposner]

@derekperkins great suggestion!

We already have the necessary filter implemented to handle this (and pass dynamic values like the post ID, build ID, etc.) into it - now we just need to figure out what works best in terms of UI/UX.