Support import of VM and TM exports

[arantius]

Both Violentmonkey and Tampermonkey support backup to ZIP. They both contain .user.js files plus platform-specific settings and/or stored values.

We should support importing the scripts (plus downloading any other referenced files, since they're not included) from both formats.

[arantius]

1. Start nightly
2. Install Violentmonkey
3. Install a couple user scripts
4. Uninstall Violentmonkey
5. Install greasemonkey-2018.05.14.nightly.4bf45ef.xpi
6. Import that .ZIP

Fail:

Security Error: Content at moz-extension://fad61490-d9a3-4ab2-be5d-6b8031489531/src/content/backup/import.html may not load or link to file:///fancybox/jquery.fancybox-thumbs.css.
uncaught exception: undefined
Error: Download(s) failed; see `.failedDownloads`.

We don't know the download URL for this script, and it specifies relative imports, which fail to resolve.

Repeat, but install scripts with no relative-URL remote dependencies: Works OK.

I think this is enough for now, but added #2952 for doing even better.