Add ButtonBlock and CalloutBlock to DEFAULT_BLOCKS

[rchlfryn]

Summary

ButtonBlock and CalloutBlock are the only two blocks not included in DEFAULT_BLOCKS. Should they be added for consistency, or is there a reason they were excluded?

Button blocks likely should not be included as standalone layout blocks.

Acceptance Criteria

• Decide whether to include ButtonBlock and CalloutBlock in DEFAULT_BLOCKS
• Ensure both blocks look good as layout blocks before adding
• Add them if appropriate