Add method to mark message as read to REST API

When exposing unread messages counters in
!1283, I
noticed that they were never set to 0.

Fixes #1780
1 job for 1780-mark-as-read in 9 minutes and 33 seconds (queued for 6 minutes and 25 seconds)
Status Job ID Name Coverage
  Test
passed #6211
test

00:09:33