Allow adding xpub keys to buttons #776
Description
Effectively allow "subscribing" to an xpub, rather than requiring the user to enter a bunch of addresses generated by an xpub.
Due to how we need to be subscribing to a lot of addresses to avoid missing payments due to "gaps" between addresses with payments, we can make this a "pro" feature. We can scan ahead ~100 addresses or so and make it clear somewhere in the UI that this is what we do.
Related to #330.