Use Mercury Editor to edit custom content blocks

In 2x we added support for editing custom content blocks in the Mercury Editor interface, similar to the way other custom entities can be edited. To my knowledge, this feature has had little-to-no use. If we're going to include it moving forward, we need to carefully think through the UX. Unlike pages (or most other entities), blocks are often used across multiple screens / sections of website, and likely need a very different UI than other ME use cases. For now, it is being deprecated and will soon be removed from the main 3.0.x-dev branch.