When removing a custom content block from a layout, offer to add it to the section library

Problem/Motivation

"Remove block" from layout warns people that this cannot be undone, but does not really make the distinction between removing a block that can be placed again and removing a custom content block where all the custom content will be lost.

It would be pretty cool for there to be a prompt at this point to save the block for later use. Not sure Section Library is where that makes sense but it's the closest looking home for this sort of feature so far.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes