Update our 2018 release schedule wrt. new Firefox ESR plans

Originally created by @intrigeri on #15092 (Redmine)

Next ESR will be Firefox 60. https://wiki.mozilla.org/RapidRelease/Calendar was updated.

Impacted:

• RM shifts
• sponsor deliverables
• anonym who’ll do the work on #15023 (closed)
• intrigeri who’ll review #15023 (closed)

To update:

• tails-dev@ and workers for sponsor deliverables
• https://tails.boum.org/contribute/calendar/
• Redmine target version dates

Related issues

• Related to #15023 (closed)
• Blocks #13245 (closed)