Update our 2018 release schedule wrt. new Firefox ESR plans
Next ESR will be Firefox 60. https://wiki.mozilla.org/RapidRelease/Calendar was updated.
- tails-dev@ and workers for sponsor deliverables
- Redmine target version dates
Calendar: start updating wrt. next Firefox ESR being 60 (refs: #15092).
For now I'm only changing what is certain and flagging XXX what will most likely