Custom block creation form redirect is inconsistent

Updated: Comment #0

Problem/Motivation

• Following #2062439: Provide listing of custom block entities → and #2058321: Move the 'place block' UI into the block listing → , there are two entry points for creating a new custom block: The custom block listing at admin/structure/custom-blocks, and the block placement form at admin/structure/block.
• When you add a custom block from admin/structure/block, submitting the custom block creation form takes you directly into the "place block" form for an instance of this new block, and then subsequently redidirects back to the place block form.
• When you add a custom block from admin/structure/custom-blocks, the behavior changes based on whether you have only one custom block type:
  • If you have only one custom block type, you are redirected back to admin/structure/custom-blocks, as you'd expect.
  • If you have multiple custom block types, you first select your block type, then fill out the custom block form... and then are unexpectedly redirected into the place block workflow and then to admin/structure/block, despite that you might not have any intention of placing the block in the first place.

Video illustrating this adventure:
https://www.dropbox.com/s/b15zhahytrww9sn/custom_block_redirect.mp4

Proposed resolution

Always redirect back to admin/structure/block/custom-blocks if that's where you start.

Additionally, if you started on a theme-specific page like admin/structure/block/list/seven, you will be redirected there.

Remaining tasks

Remove the seven theme override of the custom block list theming (ie this works in stark)

User interface changes

The user is consistently redirected to admin/structure/custom-blocks if that's the starting point.

API changes

Probably none.

Related Issues

• #2062439: Provide listing of custom block entities →
• #2058321: Move the 'place block' UI into the block listing →
• #2062715: [META] Many UI/UX issues with custom blocks. →