Replace checkers for Nocc with validators
Add the following validators / insertables:
- Printf validator for site / code
- Brace balance validator for site
- Insertable for HTML tags in site
Bug: T237918
Depends-On: I495b300e73022436f2e5f30123791b3943aa782b
Change-Id: I2dac1a3cdc5d6f02b21737771f7c4e9146c7c60c