Add preferences for EBird vs. Birdstack

[Adam Winer]

Original [issue 10](https://code.google.com/p/scythebill-birdlist/issues/detail?id=10) created by adam_winer on 2010-11-05T00:53:35.000Z:

Most users use at most 1 of these websites, if any. Add a user preference to control which is visible.

[Adam Winer]

Comment [#1](https://code.google.com/p/scythebill-birdlist/issues/detail?id=10#c1) originally posted by adam_winer on 2012-06-18T05:20:43.000Z:

Obsolete: BirdStack is shuttered, the option removed from Scythebill.