eqosys: in-app account deletion and Sign in with Apple, for the App Store #2879

Part of #2875. App Review requirements that land on eqosys, the shared login:

• Account deletion (guideline 5.1.1(v)): any app that creates accounts must let people delete them from inside the app. Add a delete endpoint and page to eqosys that each iOS app can open.
• Sign in with Apple (guideline 4.8): eqosys is email and password today, which is exempt. Once the GitHub or Google buttons are turned on, an iOS app using eqosys must also offer Sign in with Apple (or another private-email login). Add it as a provider; its keys are a MANUAL RUN for the owner.
• Demo account (2.1): a fixed review account per app, so App Review can sign in.