Pull `coinDecimals` from BitGoJS.statics

[cooncesean]

Overview

If you look at sign.js, it defines a configuration constants for each coin that the library supports.

These constants are already defined in the upstream library, BitGoJs; there is no need to create two sources of the truth.

The task: Update this library to leverage the existing constants in BitGoJS.statics.

Definition of Done

• Refactor all coins to fetch their constants from BitGoJS.statics.
• All tests should still pass after these changes.

Extra Credit

Are there any other constants in this library that have already been defined in BitGoJS.statics? If so, please add or create a new issue to track it -- if not, do you see any constants that might be useful to move upstream?

[wellkochi]

#65