Replace PostScribe in the PUC

[bretg]

The Prebid Universal Creative has been using the PostScribe library for rendering in mobile for many years. PostScribe is deprecated.

This is the relevant code in the PUC is https://github.com/prebid/prebid-universal-creative/blob/master/src/postscribeRender.js

We need someone who understands mobile webviews to step up and help Prebid figure out what's going on here.

Options:

1. PostScribe was a nice-to-have that we can live without. Just remove it from the PUC and write directly to the iframe body with javascript.
2. No, webviews really do need an asyncronous way to write or it will negatively impact the user experience. We need to update the PUC to replace PostScribe with ____.

@alexsavelyev - please discuss with the mobile committee, which owns this part of the PUC.