Remember added custom token details across accounts

**What problem are you trying to solve?**
When I add a token to one of my accounts (addresses in my den) that MM doesn't support out-of-the-box, I need to mine the data from Etherscan. I need to do this again whenever I switch to a new account and want to use the same token.

**Describe the solution you'd like**
Once a custom token is added to an account, store its details locally and treat it as an out-of-the-box supported token for all other accounts. If this is not possible, at least make it addable through searching for the symbol.

The issue names no files, tests, or entry points. Start by locating the custom-token add flow and the local data used when switching accounts; confirm that one account's token details are available to the others, with symbol search as the fallback. Done means the same custom token no longer requires repeated Etherscan lookup.

Written by the indexing model from the issue text.