Add "Select Network" screen after you create/import a new wallet

**Problem:**
During user tests in Amsterdam, I found that users don't really know how to enable their favorite networks. They don't even understand that we support multiple networks.

**Solution:**
Add one more screen where they can select multiple networks from the existing base (we make a long list of all 32 supported networks). If they would need a custom one, then they could do this later.

**Why?**

- first, we let users know we support multiple chains
- the wallet is already customized, so they don't have to go to the Settings tab

Invision Prototype:
https://invis.io/RA12NU40P8SW

**Zeplin:**
https://zpl.io/kDNl9OW
https://zpl.io/6NRPwZg