Build the Pattern Directory back-end

[tellyworth]

The back-end for the block pattern directory should be broadly modeled on the plugin directory, and can probably import some of the code.

Tentative assumptions:

• Patterns should be stored in CPTs (Stub basic post type and API endpoint #6).
• Each pattern would have metadata such as author, title, description, tags, language, screenshots. Partially done in Stub basic post type and API endpoint #6.
• Patterns: Use revisions to track pattern changes #279
• There should be an API endpoint for searching and fetching patterns. (Stub basic post type and API endpoint #6)
• Pattern searching should use ElasticSearch in a similar fashion to the plugin directory (Search description instead of post content #28).
• Some kind of curation or moderation will needed.
• Some kind of user-submitted ratings or voting will be needed.
• Some kind of popularity stats will be needed. (Track stats on installed patterns #24)

This is an umbrella task, separate issues should be created for the various parts.

[iandunn]

I took the liberty of editing the issue to make it a checklist, and reference other tickets. Feel free to revert if you don't think that's helpful.

[ryelle]

The only item on this list that doesn't also have an issue is the "pattern moderation" item. Since we're not accepting submissions for initial launch, I don't think this needs to apply (and also might be captured under #22). I'm going to close this tracking issue, the other tasks can be tracked in their respective issues.