Pledge towards any issue with Chrome Extension

[birkjernstrom]

Feedback from maintainer: Allow the Chrome Extension to embed the funding badge on all issues so they can pledge – as a backer – against anything on GitHub easily.

We support pleding against anything today (polar.new). However, we need to also improve how we prompt maintainers about such pledges for issues not proactively badged – giving them control to easily accept/deny.