Add ElectrumX server settings

[QuickMythril]

This adds two new settings for each of the supported foreign coins. These settings allow users to specify ElectrumX servers to use (or wallet servers for Piratechain), and whether or not to use the default servers provided by the Core. This gives users more control over their node, and allows easier testing for issues with specific wallets or servers. As an example, the following lines could be added to settings.json to disable default servers and connect only to qortal.link:
"useDogecoinDefaults": false, "dogecoinServers": ["electrum.qortal.link:54002,SSL"]

[kennycud]

The code that parses the settings is identical for each coin and should be encapsulated in a utility class in another file.

[QuickMythril]

The code that parses the settings is identical for each coin and should be encapsulated in a utility class in another file.

Sure, that would be a nice improvement, but it's not necessary to do that before adding this feature. If someone wants to clean up or consolidate the code at some point in the future they can do that any time, as they always have been able to. I tried to do that before with AlphaX's code for the wallets and address books and was told not to change anything. This seems like a difference in code style guidelines between the members of the official "Qortal Development Group" which doesn't involve me anymore.