Check if hash matches data stored

[zahhar]

When reference_hash is provided together with reference attribute, it needs to be validated against the content.

reference is optional:

• if empty - do nothing
• if absolute URL, read it and expect JSON, link to it
• if not URL, try to download base_uri+reference, expect JSON and link to it
• if smth else (not JSON payload or not 200OK) - show warning

reference_hash is optional:

• if empty - do nothing
• if present, but reference is empty - warning
• if present together with reference - display it, and validate.

reference_hash validation:

1. Download the content from reference (same will be for media in the Token context later)
2. Calculate and compare if any plain hash matches: base64-encoded sha-256, sha-384 or sha-512
3. If not, assume given hash is Multihash, and try to check it using https://github.com/multiformats/js-multihash
4. Error if nothing worked - probably referred data was tampered