* Currency: Do not use a default forex provider
exchangerate.host now requires an API key.
Instead of finding a new Free (for now) default, this change simply
disables the currency exchange when nothing is enabled.
* SyncManager: Report ?.?? for an unknown forex amount
In a situation where we thought forex was available but we got an error,
this avoids showing 0.00 when there was actually an error.
* Currency: Tests for no default forex
* Currency: Use mock provider for tests
* Currency: Add API key to exchangerate.host
* Currency: Remove Exchangerate.host
Exchangerate.host was bought by apilayer, the old API deprecated, and
replaced with a proxy to the apilayer api.
We already have currencylayer support, so ther's no reason to keep exh.
Worth noting: New ERH keys actually work on currencylayer
* Currencies: Add test coverage for currency layer
* fixup! Currency: Tests for no default forex
Remove duplicate assignment
Fixes [review comment](https://github.com/thrasher-corp/gocryptotrader/pull/1395#discussion_r1395178513)
* fixup! Currency: Add API key to exchangerate.host
Remove unused ErrVar
Fixes [review
comment](https://github.com/thrasher-corp/gocryptotrader/pull/1395#discussion_r1396647418)
* fixup! Currency: Tests for no default forex
Fix spelling of override in test
Fixes [review comment](https://github.com/thrasher-corp/gocryptotrader/pull/1395#discussion_r1396701476)
* fixup! SyncManager: Report ?.?? for an unknown forex amount
Fix display of non-positive currency conversions.
Fixes [review comment](https://github.com/thrasher-corp/gocryptotrader/pull/1395/files#r1398527134)
