build: Upgrade eslint-config-wikimedia from 0.17.0 to 0.20.0

A number of fixes from new upstream rules. Also, bump npm audit
issues from browserlist and postcss, and make the ES version
restriction apply appropriately rather than to the source code..

Change-Id: Ic9548930a172d06fff97d7c59d4f8a1e5b79faeb