Create notifications for "update: start" push events

[jeffposnick]

In generateSessionNotification(), we're only currently creating notifications for "update: details" events, i.e. something about a session's metadata changed.

We need to add in logic that will display "update: start" notifications that are tied to the start of a session, including the keynote.

[x1ddos]

@jeffposnick if backend sends update: start for __keynote__ a day before, will you be able to distinguish between "io starts soon" and "session starts in 10 min"?

Maybe the best for sw is to look at the actual startTimestamp of the session and format the notification message accordingly?

[x1ddos]

Actually, @jeffposnick does it work for you if backend sent update: start and update: soon, where soon means like 1 day before? You can also check startTimestamp to get exact timing anyway.

It would be cool, I think, if you displayed "X session starts in Y minutes" for update: start, where Y is calculated at the time you're processing updates in the sw, e.g. startTime - new Date().

[jeffposnick]

I believe "X session starts in Y minutes" will be misleading for folks who don't see the notification right away.

We can hash out the wording details in the PR for the client-side changes.

[x1ddos]

yeah, it was just an idea.