Firefox 60 deprecation

[cadeyrn]

With the release of Firefox 70 and Firefox ESR 68.2 in October 2019 Mozilla won't support Firefox ESR 60 any longer, therefore the Firefox 60 support in Enterprise Policy Generator will be deprecated with the release in October 2019.

• bump minimum required Firefox version from 60.0 to 68.0
• change the following code:

  enterprise-policy-generator/src/js/core/configurator.js

  Line 1038 in 22c016c

  if (policy.first_available.mainstream !== '60.0' || policy.first_available.esr !== '60.0') {

• don't forget to add a note to the CHANGELOG about the new requirement and to update the README
• remove versions notes for policies older than Firefox 68 from translation files
• change explanation_about_policies and policy_description_Certificates_ImportEnterpriseRoots translations