export / import CLI tooling

[tsmethurst]

We should add some functionality for exporting and importing instance data, to facilitate the following scenarios:

1. An admin wants to move their instance from one database type to another -- they should have a simple way of doing this without losing data.
2. An admin wants to migrate from one pre-release database schema version to another, without having to do a total wipe of their data -- this is especially relevant given the schema changes made in Struct validation #186
3. An admin wants to close their instance but might want to start it again later and not break federation by losing public + private keys of instance accounts.

A good place to add this functionality would be in the gotosocial CLI tool, under the admin command paths.

For example, we could have something like gotosocial admin export with parameters for specifying only local accounts, all accounts, etc, then gotosocial admin import. These commands could export to/import from a JSON file.

Minimum viable export/import

To make export + import across database migrations work, we'd need to store the following:

• Account -- store all local accounts, and store remote accounts that they follow/are followed by/block/are blocked by.
  • ID
  • CreatedAt
  • UpdatedAt
  • Username
  • Locked
  • Language
  • URI
  • URL
  • InboxURI
  • OutboxURI
  • FollowingURI
  • FollowersURI
  • FeaturedCollectionURI
  • ActorType
  • PrivateKey -- for local accounts only
  • PublicKey
  • PublicKeyURI
  • SuspendedAt
  • SuspensionOrigin
• Block -- all fields
• DomainBlock -- all fields
• EmailDomainBlock -- all fields
• Follow -- all fields
• FollowRequest -- all fields
• Instance -- all fields
• User -- all fields

In this case, all statuses, emojis, faves, notifications, tokens, attachments, clients, and applications would be dropped.