Import/export CAR DAG archives

[lidel]

Given that:

• ipfs dag import|export exists
• go-ipfs 0.9+ supports exporting DAG archive via any public gateway (/api/v0/dag/export)
• tools like ipfs-car and ipfs-content-addressed-archiver make it easier for people to produce/consume content-addressed archives

We should support "DAG import/export" in ipfs-webui somehow.

Import DAG

Import could work the same as "From IPFS" :

We most likely want to add an explicit "From CAR Archive" item here (personally, I like the idea of a whimsical car icon :)), but "stroke data" from ipfs-css would also do.

Export DAG

Add complimentary "Export DAG" to context menu on Files screen:

[Gozala]

Seems like it would be a good idea to do it after ipfs/js-ipfs#2745

[hacdias]

@lidel as I mentioned on #1838 (comment), we'll need to update to the latest HTTP client across WebUI and the Explore page to support the import part. The export is done on #1837. I don't think I should work on the upgrade right now as that would take a long time working only 10h a week. Updating the http client will also require updating everything else connected: multiformats, ipld, multicodecs, etc.

I think we could merge the export button and evaluate how much effort is needed for the import part.