Merge branch 'rename-metadata-key' into 'master'

Rename metadata key for clarity
Compose bounce messages so that they can be properly translated

See merge request !304
6 jobs from master in 29 minutes 46 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #27783962
docs

01:49

passed #27783965
mysql
mysql

17:39

passed #27783964
postgres
pgsql

15:49

passed #27783961
qa

01:56

passed #27788547
sqlite

12:03

failed #27783963
sqlite

13:42