Closes #1871 

This PR adds the capability so that the onboarding commit message (“Add renovate.json”) will have a prefix if semanticCommits is explicitly set to true by the bot administrator.

e.g. “renovate a/b —semantic-commits=true —semantic-commit-type=foo” will result in the onboarding commit being “foobar: add renovate.json”.

Closes #1867

This PR refactors `branchName`, `commitMessage` and `prTitle` so that they are more easily editable and hopefully more understandable. By breaking each up into subsections, users can modify one part without needing to copy/paste the entire string.

Directly editing any of these fields will now be deprecated and a warning issued.