Add ability to export/import DIDs

[rmccue]

Introduces an "export" button on DIDs, which gives a JSON document that can be reimported. This document contains the private keys stored in the database (in multibase multikey encoded format).

Fixes #7. This is based off the #14 PR, so that needs to be merged before this is - I can rebase afterwards.

Example export, did-abc123def456abc123def456.json:

{
    "@context": "https:\/\/fair.pm\/did-export",
    "_warning": "WARNING: This file contains your private keys for your DID. Do not share it publicly, as it will lead to your DID being PERMANENTLY COMPROMISED and UNRECOVERABLE.",
    "id": "did:plc:abc123def456abc123def456",
    "rotation": [
        "z6Dxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    ],
    "verification": [
        "z6Dxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
        "z2bxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
        "zruxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    ]
}