This pull request seeks to refactor default block by post format to be effected as a template.

This is being proposed for two primary reasons:

• It is a simplification, reusing existing template mechanisms in place of an additional step for enacting the default post format block
• As an extension of the previous point, it will serve to benefit a planned subsequent refactoring of editor initialization aimed at eliminating "setup" steps of the editor, for which the default post format block is otherwise difficult to mimic in state without a designated start point.

There are two notable differences from how this behaves in master:

• The template is not enacted except when editing a new post.
  • Arguably, I find this behavior, both for the default post format block and templates in general, to be buggy. For example, on master try:
    1. Set default post format to Image on Settings > Writing
    2. Navigate to Posts > Add New
    3. Add a title
    4. Remove the default image block
    5. Save the post
    6. Reload the editor
       • Expected: My changes are reflected (i.e. the removal of the default block)
       • Actual: The image placeholder block returns! Again, I acknowledge this could be an argued as intentional, but to me it seems like the default block should serve as a starting point niceity, but ultimately we respect the user's decisions to manipulate or remove those defaults.
• The default block is no longer inserted when changing Post Format field
  • Another niceity, but I don't find it to be a worrisome change

Testing instructions:

Verify there are no regressions in the behavior of the default block by post format.

Ensure new end-to-end tests pass:

npm run test-e2e