notification icon in header

[rbuels]

Add a notification icon that displays "messages" that come from the JBrowse developers.

Would download a JSON file from jbrowse.org that contains messages structured as:

[
   {
    id: "unique_id",
    messageHtml: "HTML content of the message we want to send to all jbrowse users"
   }
]

the button would keep local state (in localstorage or similar) in order to know whether a given message ID is "new". If one or more messages are "new", will have a little attention dot on it to call attention to the message button.