Issue 8.x-3.6 release

We're currently short of maintainer time for Features and so have been slow to address bug fixes and maintenance. We're especially short of testing expertise, with the result that several patches have languished in the issue queue for lack of accompanying tests or for lack of expertise in troubleshooting test failures.

There are several issues in the queue that should go into a new release.

Probably the most important is #2913651: Drush 9 Incompatibility. That's nearly ready, just needs a minor tweak and clarification about implications for users of past Drush versions.

Other bug fix issues have patches but no accompanying tests or test failures. This is the queue.

Help getting some of these bug fixes finished and applied prior to a release would be very much appreciated.