Next version will be Etherpad 1.8. As planned in #3424, we are going to require
NodeJS >=8.9.0 and npm >= 6.4.

This commit implements that change and updates documentation and scripts.
Subsequent changes will get rid of old idioms, dating back to node < 0.7, that
still survive in the code.
Once migrated to NodeJS 8, we will be able to start working on migrating the
code base from callbacks to async/await, greatly simplifying legibility (see
#3540).

Closes #3557