Add support for publishing repo to decentralized storage (was IPFS via Web3.storage)

Fdroid repos should be able to be published or mirrored to IPFS using the new Web3.storage method and service: https://web3.storage/ It is "free", likely as a loss leader to promote the use of Filecoin, but for now you can push up to 1TiB of storage per account.

Once available on IPFS, the repo can be accessed using a public HTTP-to-IPFS gateway, so as not to require fdroidclient to directly support IPFS (yet). More info on that: https://docs.ipfs.io/concepts/ipfs-gateway/

In the end, you should have a mirror or repo path that looks like: https://gateway.ipfs.io/ipfs/ABC123/fdroid/repo

where ABC123 is the content-ID/hash for the published IPFS content