Adopt xcconfig, improve development experience for external contributors

[insidegui]

This merges xcconfig adoption changes from #669 and makes the following changes:

• Introduces a new OpenSource.xcconfig file that's used for the WWDC scheme in both Debug, and Release configurations (i.e. the scheme without iCloud support)
• When building from that scheme, the bundle ID gets the development team ID appended to it, just like with Apple's sample code
• A new TeamID.xcconfig file controls the development team ID, this file is not included in source control
• Running the bootstrap.sh script (or just the teamid.sh script) will ask for the team ID and generate the TeamID.xcconfig file automatically

This way folks can pull the repo and get up and running quickly without creating spurious file changes that get picked up by source control due to setting code signing things manually in Xcode's UI.

[insidegui]

Confirmed to be working correctly on my machine!