Call for testing: feature/buster (May 2019 edition)

At the end of the ongoing Buster sprint, we should send a call for testing to -testers@ and Twitter, about:

• any regression
• #5463 (closed) (done in feature/buster already)
• #14991 (closed) (done in feature/buster already)
• possibly #14580

Related issues

• Related to #5463 (closed)
• Related to #14991 (closed)
• Blocks #16209
• Copied to #16755 (closed)

Original created by @intrigeri on 16749 (Redmine)