Adds the effects necessary for supporting reusable blocks. There are 4:

- FETCH_REUSABLE_BLOCKS: Loads reusable blocks from the API and inserts
  them into the store.
- SAVE_REUSABLE_BLOCK: Persists a reusable block that's in the store to
  the API.
- MAKE_BLOCK_STATIC: Transforms a reusable block on the page into a
  regular block.
- MAKE_BLOCK_REUSABLE: Transforms a regular block on the page into a
  reusable block.

This unnecessary line probably came as a result of a bad merge conflict
resolution.

`gutenberg_collect_meta_box_data` expected that
`window._wpGutenbergEditor` would always be instantiated, but this is
only true for when `wp.api.init()` returns quickly because of the API
schema being cached.

The fix is to use a Promise to store `window._wpGutenbergEditor`.

(Well, a jQuery.Deferred, since we have to support IE 11 which doesn't
support promises.)

Remove jQuery.Deferred and jQuery.when in our editor initialization in
lieu of Promise and Promise.all.

Make it clear that this variable is a promise by giving it a non-noun
name.