Maker has upgraded its Dai token to "Multi-Collateral Dai" (MCD) and requires
all users interacting with Dai migrate their tokens to the new version. Dai
now exclusively refers to Multi-Collateral Dai and what was previouly called
Dai is now Sai (Single Collateral Dai).
In this description, Sai refers to what was (prior to the 2019-11-18) known as Dai.
Dai is the _new_ token.
This changeset:
1. Only affects users who had non-zero Sai at the old contract address
2. Displays a persistent notification for users with Sai
3. Updates the token symbol for users already tracking the Sai token
4. Bumps our direct and indirect eth-contract-metadata dependencies
The notification copy:
> A message from Maker: The new Multi-Collateral Dai token has been released. Your old tokens are now called Sai. Please upgrade your Sai tokens to the new Dai.
The copy is from the Maker team.
