MetaMask switch network banner (MORE FIXES)

[Steedie]

What

1. Changed banner from relative to fixed.
2. Moved the banner to the bottom of the screen.
3. Added a close/hide button to the banner.
4. Added slight blur effect.

Why

1. relative caused the rest of the UI to be pushed down in most cases.
2. Jimmy suggested moving it to the bottom of the screen. Now that it's using fixed, it overlays the rest of the UI. This is a problem when the banner is at the top of the screen because it overlays the Account Settings button. As it's at the bottom of the screen, it doesn't block the more important things like that, and zone info etc.
3. Since it can still block some buttons, I think it makes sense to provide an option to hide the banner without being forced to switch networks.