extract reconnect-notice as module

[braid-sync]

A task for new Braid contributors.

We're extracting parts out of "Braid core" into loosely-coupled modules, to ease future development. As part of this effort, the functionality of the "reconnect-notice" can be extracted.
Currently, when the websocket is disconnected, a red notice bar is shown (you can trigger this by starting Braid in dev, then, in your repl, doing (stop!)).

Extract this functionality out of core and into a new module.

Notes:

• braid.core.client.ui.views.reconnect-overlay is where the view is now
• refer to braid.stars for how to write a module
• you should make use of braid.core.api/register-root-view!

[jamesnvc]

Completed by 2004f