Token list refresh

[aristidesstaffieri]

We currently use Stellar Experts top 50 token list as a way to warn users when they interact with tokens that are not trusted by this list.

We want to move to a model that is not derived from those metrics to allow inclusion of tokens like Paxos' USDL before seeing much use in the network.

One option is to adopt https://github.com/soroswap/token-list as a new default and collaborate to add new tokens
Another option could be to maintain our own list.

In a related effort, we need to find a way to support icons for custom tokens, can this list serve as a source for icons for the tokens on the list?