[META] [UX] Improve the overall creation and placement of custom blocks experience.

If we implement #1276 (I think we should), we'll have the situation where advanced users know where to go and what to do with the created block. Not sure about newcomers though...

Previously, they would reach the create custom block form through the "Structure" menu. So chances are they've already noticed the "Layouts" menu item there and I guess they are more likely to go back looking for something related there that will point them to a part of the admin UI where they can use/place their new block. Anyways, this is about to change slightly - no biggie

So upon creation of a custom block, the user is taken to the list of custom blocks and there is a "custom block created" message. The operations available for each block in that listing are "configure" and "delete". There is room for UX improvement here: 1. [Disclaimer: crazy idea!!] Perhaps add a "Add to a layout" action to the drop-button for custom blocks in the listing page (make it default?). This would open a "Select layout" dialog. Once the layout is chosen, the user is taken to a variation of the layout admin UI where the "Add block" buttons in each region are replaced with "Place block-name block here" buttons instead. Once the block is placed in a region, the layout admin UI switches back to "normal" mode and user can add more blocks or re-arrange as they see fit. 2. Add something along the lines of "The new block is now available to be placed in any [layout](link-to-the-layout-admin-ui)." to that message. If we end up implementing my crazy idea above, we have the same steps available here (select layout -> "special" place-the-block layout UI variation -> place block in a region using the "Place block here" button -> switch to "normal" layout admin UI). 3. Going the other way around, we could place a "Create new custom block" link in the "Add block" dialog of the layout admin UI. 4. Convert the "Add block" button of each layout region to a drop-button with the default action still being "Add block" (opens the modal) but also add a secondary "Create new custom block" action. That would take the user to the add custom block form and upon block save, the block would be added to the layout region of the layout he was editing and the user redirected back to the respective layout edit form where they can re-arrange the newly added block. 5. Would be nice to be able to filter the list of blocks on the "Add block" dialog by system/custom ones (checkboxes). It was ok when I tested with a single custom block (started typing its name on the search field), but the list of available blocks might get overwhelming if you have a considerable amount of them. 6. Would also be nice to have a "Used in" column on the custom blocks list that shows layout/region information. This could be a link to the respective layout edit UI. Guessing there could be multiple layouts where a block might be used, so this way people can jump there.