chore: enforce Netlify style guide in CLI

[erezrokah]

Per this comment #1836 (comment), we should find a way to enforce our style guide in the CLI output.

Some work has been done to extract log messages to a centralized location:
https://github.com/netlify/cli/blob/33fc7fbc068a825c14730047162900ca93737044/src/lib/log.js

We should add any new message to that file and move existing ones.
Having all log messages in a centralized location will make it easier to lint them per the style guide.