[NEW] Credits screen for contributors

[cardoso]

I plan on releasing this to the app store at some point (maybe after October, when we have a few useful features), so it would be nice to have a screen with all contributors.

I might take on this issue myself, but feel free to take it.

Requirements:

• SwiftUI screens: ContributorList, ContributorProfile
• Contributor list as local JSON file
• Localized strings
• ContributorProfile: Gravatar profile pic, e-mail, Links to GitHub, LinkedIn, Twitter, etc... (all optional depending on what's in JSON)
• Continue using Redux pattern as much as possible.