Overwrite changes in config files unless blacklisted

What does this Merge Request do?

Changes the behavior of bin/safe-sed and the ERB renderer. Unless the user has added a glob pattern to gdk.yml saying a file should not be overwritten, always overwrite it.

gdk:
  protected_config_files:
    - 'gitaly/*.toml'

This removes the old overwrite_changes setting.

I think this change is better than the status quo because most users just want the latest config. The existing system of nagging users for 5 seconds makes no sense (I think people don't understand what they are supposed to do).

However, because this will inconvenience those users that do have custom config, let's ship this first in a way where it behaves like before (no overwriting) so we can give those users the chance to update their gdk.yml and protect the files they care about.

Tracking progress in #780 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.