Ask server to start sending 'submessage' events

This is the beginning of implementing zulip#3205

We currently do not do anything with the received data, but
acknowledge the existance of the `submessages` property in the
`Message` type.

Author: borisyankov

src/config.js

@@ -59,6 +59,7 @@ const config: Config = {
     'realm_filters',
     'realm_user',
     'stream',
+    'submessage',
     'subscription',
     'typing',
     'update_message',

tools/spellcheck.eslintrc.yaml

@@ -138,6 +138,7 @@ rules:
     - stacktrace
     - str
     - subheader
+    - submessage
     - subtree
     - td
     - th