Prefill ReplyEditor with some mentions

If replying to a root content, prefill that author.
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Refs: #574
3 jobs for reply-improvements in 16 minutes and 51 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Lint
passed #6837
lint

00:04:11

 
  Test Backend
passed #6838
test_backend

00:07:46

 
  Test Frontend
passed #6839
test_frontend_unit

00:04:52