Make the settings consistent with MinervaNeue:

- Enable ESLint caching with `--cache` which presumably improves
  performance.

- Forbid warnings by setting `--max-warnings` to zero. Code containing
  warnings should not be committed. However, warnings are still an
  exceptionally useful distinction to make from errors during
  development. When hacking, we do not care if a comment exceeds the
  maximum line length or if a trailing space is present but we do oh so
  very much care if the linter detects a likely programming error such
  as forgetting to initialize a constant. The former is a warning and
  the latter is an error.

- Forbid unused lint directives by enabling
  `--report-unused-disable-directives` and fix offenders. This setting
  prevents outdated ESLint error waivers from littering the code.

There is a related pull request to move these settings to defaults in
eslint-config-wikimedia itself:

    https://github.com/wikimedia/eslint-config-wikimedia/pull/82/files#diff-46af3d30ba7affc4adf37ef4c5382c39

Change-Id: I9ac69c6f7e64be6709a569ac1c5932cc28aab795