base.html: Add a new ReconnectingEventSource

base.html: Add a new ReconnectingEventSource

We originally implemented this in lmno.js in the client, but realized
it needs to be present in base.html.

This wrapper for EventSource will attempt to reconnect after a lost
connection (up to 5 times with a 2-second delay between each attempt).

With this in place, a quick server restart (< 10 seconds) should be
unnoticed by clients.