Pattern CPT & API: Add block types meta field

[ryelle]

See #33 — this adds a meta field for block types, which is exposed in the API as meta.wpop_block_types. This is an array of block type names. Once this is added to the CPT and deployed, it will become available to http://api.wordpress.org/patterns/1.0/. A corresponding PR will be made in gutenberg to add support to the /__experimental/pattern-directory/patterns endpoint.

I went with a meta field for now, since it can easily be set (in UI or WP-CLI) for the existing core patterns, and doesn't need the full infrastructure of a taxonomy. If we decide once building the UI that a taxonomy makes more sense, we can write a script to convert the post meta (but I think meta will be the way to go).

How to test the changes in this Pull Request:

1. Add a custom field called wpop_block_types to a pattern. The value must be a block type name, formatted like namespace/name - it should strip any non-alphanumeric characters out (allowing / and -).
2. GET /wp-json/wp/v2/wporg-pattern/[ID] — the meta value should include the value you entered.
3. GET /wp-json/wp/v2/wporg-pattern/ — all patterns should have a meta.wpop_block_types value, defaulting to empty array.

Try adding various values to the meta field, add multiple, etc.

[iandunn]

Left a few questions, but fine to merge as-is.