Handle multiple line names

Add function to get the display name for an author on a single line as nothing stops there being linebreaks.

BUG:476731

autotests/data/test-eventhandler-sync.json

@@ -135,6 +135,21 @@
         "unsigned": {
           "age": 1234
         }
+      },
+      {
+        "content": {
+          "displayname": "Look\nat\nme\nI\nput\nnewlines\nin\nmy\ndisplay name",
+          "membership": "join"
+        },
+        "event_id": "$143273582443PhrSh:example.org",
+        "origin_server_ts": 1432735824659,
+        "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+        "sender": "@newline:example.org",
+        "state_key": "@newline:example.org",
+        "type": "m.room.member",
+        "unsigned": {
+          "age": 12345
+        }
       }
     ]
   },
@@ -373,6 +388,20 @@
         "unsigned": {
           "age": 1238
         }
+      },
+      {
+        "content": {
+          "body": "A message from someone who thought it was a good idea to put newlines in their display name.",
+          "msgtype": "m.text"
+        },
+        "event_id": "$153456889:example.org",
+        "origin_server_ts": 14327358246589,
+        "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+        "sender": "@newline:example.org",
+        "type": "m.room.message",
+        "unsigned": {
+          "age": 1230
+        }
       }
     ],
     "limited": true,