[keyserver] use default channel cover image stored on in keyserver/images in frame
Summary:
Instead of using the link to the warpcast's default channel cover image, we store it on the keyserver and serve it at a frog hono route to be used by the frame's `<img /> JSX element as the src
Depends on D13998
Test Plan:
Verified on the warpcast frame validator and confirmed that specifying a channel with no custom channel cover still resolved to the same default cover image
Reviewers: ashoat, varun
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D13999