Resolve "Add ability to choose Electrum server, ElectrumCluster"

Resolves #15 (closed)

Allow user to choose from a list of known electrum servers, or enter a custom electrum server.

If custom electrum server fails, wipe it, notify with toast & fallback to the selected known server.

Also added the ability for buttons to have a grey border and a new "consts" subfolder of utils.