Track 'offline' notifications

[pablofullana]

As a logged in user, when opening the dApp, I'd like to see notifications for all the events (relevant to myself) that occurred while I was 'offline'

[fvictorio]

There are a lot of unknowns here:

• What kinds of notifications are we going to show? Some that come to mind: one of your orders was filled (partially or totally), one of your orders expired, one of your orders was removed from the relayer because it became invalid (for example, you don't have enough tokens anymore).
• How are we going to get these notifications? One option is to save in the local storage the last timestamp the relayer was checked and the last block the exchange contract was checked. With the contract we can get all events since then that involve the current address. With the relayer, I'm not so sure. Do we have some mechanism to know that an order was removed since that timestamp?