Task messages: include severity level over GraphQL

[MetRonnie]

Needed for https://github.com/cylc/cylc-ui/issues/1276

Currently the only information is the message

  [[foo5]]
      script = cylc message 'WARNING:Hello world'

query {
 jobs {
   id
   messages
 }
}

{
 "data": {
   "jobs": [
     {
       "id": "~rdutta/JOB//1/foo5/01",
       "messages": [
         "started",
         "Hello world"
       ]
     }
   ]
 }
}

If we just include the SEVERITY: prefix in the message sent over GraphQL this ought to be backward compatible, I believe?