FEATURE: Add support for image attachments

[angusmcleod]

@pmusaraj This adds support for image attachments in the simplest way possible. This is the first step in adding media support.

Next steps after this are:

• Potentially improve the attachment - post - upload relationship. This likely will be needed for outgoing support.
• Improve image feature support, e.g. Mastodon's focalPoint), blurhash, and width/height attributes.
• Add more tests, e.g. integration (end to end).

Notes:

• Some platforms use Document attachment types for images (instead of Image).
• Attachments can be added to Objects other than Notes or Articles (e.g. Mastodon uses it for certain profile attributes), and we may want to support other types of attachments in the future (hence the need for a ploymorphic association).

[pmusaraj]

Sounds good to merge this separately from outbound attachments. Let me know when it is ready for review. (Given Draft status and conflict, I am guessing it is not yet ready.)

Thanks @angusmcleod

[angusmcleod]

@pmusaraj I've added attachment publication support to this PR. It's ready for review!

[pmusaraj]

Thanks @angusmcleod, overall this looks good to me, just one question.

[pmusaraj]

Does the spec only support one attachment per note/article?

[angusmcleod]

It supports multiple. The spec (and implementers like mastodon) use the singular term attachment, which actually an array of attachments. Yeah, I found it confusing too.