Manage MetaMask connection in store instead of component

[annybs]

Similar to #142 -

At the moment, the Deposit modal contains all state pertaining to the page's MetaMask connection. This state can be lost while navigating between pages.

Moving the MetaMask connection state (and associated values) to the global store will keep the connection alive, and also make it easier to validate & respond to changes independently of individual components. We could consider adding UI elements specifically for managing connection status, e.g. status banner, a button to forcibly disconnect MetaMask.