Add currency symbols where appropriate

[jkup]

Using a library like https://www.npmjs.com/package/currency-symbol-map it would be nice to add currency symbols on the ledger settings pages for the appropriate currency type.

It looks like "currency" is stored in state as a string (https://github.com/brave/browser-laptop/blob/master/docs/state.md#L403), so we should just be able to pipe it through a library like the one above and then prepend it to the currency number.

[luixxiul]

Test plan: #6249 (comment)

[bradleyrichter]

This needs to be changed to use ISO 4217 3-letter codes instead of symbols.

[bsclifton]

I'll get this squared away

[bsclifton]

The fix for this issue was reverted with #6720. Removing milestone from this issue + original PR.

cc: @luixxiul in case you needed to remove the release notes for this

[mrose17]

let's declare this done!

[luixxiul]

I added QA/no qa needed release-notes/exclude labels