Piece Doctor web UI: Add page to list payload cids in a piece

[dirkmc]

On the Piece Doctor page in the web UI, we should add a link called "Payload CIDs" next to the Piece CID that opens a page listing all payload CIDs in the piece. It should show a table with three columns:

1. The payload cid as a string "bafy..."
2. The corresponding multihash in hexadecimal format.
3. A link to download the block (same as the "Download block" link on the main Piece Doctor page)

To make it easier to search for a particular cid, there should be no pagination: the whole table should display on one page.

The resolver can call PieceDirectory.GetIterableIndex() to get the list of multihashes, and convert each to a cid with

cid := cid.NewCidV1(cid.Raw, m)