Make signing configurable for contributors

[lukaaas176]

After cloning the project and selecting my own development team, Xcode modifies the shared project files, bundle identifiers and entitlements.

As persons testing won't have access to the main Apple Developer account, it would be nice to configure the signing locally without having changes to the upstream project all the time and ignoring it locally.

There already is a Secrets.xcconfig for the development team, but currently it does not cover every target and the bundle identifiers/app group are still defined directly in the project, entitlements and some Swift files.

I would have the following suggestion and would be willing to implement and provide a pull request for that :)

• Make the app, tests and share extension bundle identifiers configurable
• Make the app group identifier configurable in the entitlements and runtime code
• Keep the actual developer values in an ignored local .xcconfig
• Provide default/example values for contributors

This should allow everybody to use automatic signing with their own account without modifying the shared project files.

Any thoughts? :)

[subpop]

I would happily accept a PR that makes it easier for external contributors. It's awkward for me to test it, since I do have access to the developer account, but I think @rexbron would be able to give some feedback on whether or not it works.