Update the sites/block-layouts endpoint to use the existing set of layouts and categories.

[enejb]

We want to expand the current number of mobile only layouts to include all the .com layouts. And make so that any new and exists .com layouts once added will be show up as layouts in the block-layouts endpoint.

Describe the solution you'd like
The newly created api endpoint #2413 should contain additional layouts and categories that a user can choose from.

What does Done look like?

• Use the system that already exists in wpcom to return the current layouts. We want to be able to get the new layouts or updates on .com service without putting extra effort for the mobile side.
• Filter the layouts by supported block. (Only return layouts that only have the supported blocks)
• Provide a excluded lust so that even more layouts can be excluded if so desired listed.
• Endpoint continuous to support the current preview_width and scale attributes as expected.

Things that are not included.

• Support for vertical content. (This can be re examined in the future PRs/Issues.)

[enejb]

Since we shipped D45634-code I am closing this Issue as done.