Disable button when creating new message and close modal when done

Small UI tweaks when creating new messages/comments:

- The button switches to a disabled state with the label "Sending...", until the API call finishes (this is to avoid the user clicking multiple times and creating duplicate messages if ever the API call is a little slow)
- The modal closes when creating a new note (fixes https://github.com/tidepool-org/hub/issues/101)

@jh-bate Maybe you'd like to have a quick look, tell me what you think? Thanks!