allow users to add and manage the IPFS gateways in use

Users should be able to manage the IPFS gateways that are in use, e.g. #2504 (closed). It looks like it should be possible to do IntentFilter matching since they should always use https as the scheme and /ipfs/ as the first path segment. There is currently a hardcoded !1199 (merged) list of IPFS gateways. One approach could be to move this to the fdroiddata config, then fdroidserver would include the list in index-v2 and the client would get them from there.