Refactor module providing `linter` command.

Notable changes:
* Remove unused import `django.core.management.CommandError`.
* Split path '../../..' into  three pieces. (Each argument to `os.path.join`
  should be a single path component.)
* Add `pylint: disable` directive.
* Remove `print` statements. The `print` statements competed with pylint for
  access to stdout. When two programs compete for access to stdout, output can
  be uglified, and users cannot do interesting things like pipe the results of
  pylint to `rst2html`. (Most of Pylint's output is valid reStructuredText.)
* Rewrite prose.