Add BrickList API to constrain possible child types

**Use case:**

BrickLists have a collection of bricks attached, but we wan't to constrain the types of bricks for any given brick list. Example: An AccordionBrick should only have AccordionItemBricks, a TwoColumnBrick should only have ColumnBricks.

- [ ] provide API for bricks to determine what they accept as child bricks
- [ ] provide instructions on how to globally enable/disable bricks

**Points:** 2