settings: Make DEFAULT_CONFIG public

This dictionary is accessed by plugins (like `summary`) which add new
settings, so it should be public (i.e. no prefixed underscore).

The changed name length would have led to a re-indenting of the
default contents anyway, so I shifted them all to four spaces.
settings: Rework the LESS_GENERATOR removal warning for easy extension

Setting migration is something that will happen for multiple settings,
and the more we can standardize the reporting procedure, the easier it
will be to add new warnings.
settings: Add a warning for folks using FILES_TO_COPY.