Add accessible list attributes to chat message items for easier navigation

Add accessible list attributes to chat message items for easier navigation

Context: https://gitlab.com/gitlab-org/gitter/webapp/-/issues/538#note_326609182

Part of #538

Testing strategy

Do the following steps for a room, thread, and archive:

1. Notice each .chat-item has role="listitem"
2. Notice that #chat-container has role="list"
3. Send a message and notice the new message element has role="listitem (this step doesn't apply to the archive)

Not testing with a screen reader but it's what @MarcoZehe suggested: https://gitlab.com/gitlab-org/gitter/webapp/-/issues/538#note_326609182 and looks sane to me 👍