BTC bridge uses federation protocol

Strip out all the special-case code for the BTC bridge and just use
normal federation.  This allows the default bridge configuration to be
just a domain name instead of the full quote URL.
Allow the BTC bridge domain to be configured

Expose the setting on the options page.
Merge pull request #1200 from singpolyma/custom-btc-bridge

Allow the BTC bridge to be configured by users