PLT-6481 Create platform route and logic to be able to DM a user by email or username (#6310)

* Add logic to be able to DM a user by email or username

* PLT-6481 update DM url to be in the format https://servername.com/teamname/messages/xxxxxxxx

* PLT-6481 Adding logic to get user by full email

* PLT-6481 logic for routes /messages/user_id and /messages/id1_id2 to redirect to /messages/@username

* PLT-6481 logic for GM route /messages/generated_id

webapp/utils/utils.jsx

@@ -1099,7 +1099,12 @@ export function getDirectChannelName(id, otherId) {
 
 // Used to get the id of the other user from a DM channel
 export function getUserIdFromChannelName(channel) {
-    var ids = channel.name.split('__');
+    return getUserIdFromChannelId(channel.name);
+}
+
+// Used to get the id of the other user from a DM channel id (id1_id2)
+export function getUserIdFromChannelId(channelId) {
+    var ids = channelId.split('__');
     var otherUserId = '';
     if (ids[0] === UserStore.getCurrentId()) {
         otherUserId = ids[1];