Warn of payments to exchanges without memo

We see so many users forgetting to set the memo when sending funds to exchanges like Bittrex and Kraken that we should consider displaying a warning when they try to do so. I've noticed stellarchain, stellar dashboard, and stellar expert all identify these exchanges (perhaps by home domain? or is this hard coded?).

I see dashboard has a known_accounts.js. Is this something that is useful to maintain separately and host on a CDN? As more projects need to keep a registry of accounts it's maybe a good idea to have only a single source to keep up to date.

[bartekn]

Yes, we will extract known_accounts.js and maybe create a web service around it. I think that PR adding this feature (with static list of exchanges in the beginning) will be approved.