Consider moving invite conversion logic from UI code into core as well

[Simon-Laux]

desktop, iOS and android duplicate the same logic, why not return it from core instead of the current raw OPENPGP4FPR link, or should that still be used somewhere?

https://github.com/deltachat/deltachat-ios/blob/9c3c7a754bbbc92503db380efe610d0aff781716/deltachat-ios/Helper/Utils.swift#L45

https://github.com/deltachat/deltachat-desktop/blob/3b8f0275ff8619083ae304ec86347fbc0047db88/src/renderer/utils/invite.ts

https://github.com/deltachat/deltachat-android/blob/953a8882397e9e65ee000bad325853052f91f3b2/src/org/thoughtcrime/securesms/util/Util.java#L81

[r10s]

related: we're considering to let core return always i.delta.chat-links. only the invite page would generate the OPENPGP4FRP:-links then. cmp chatmail/core#5467