Prevent schema changes to branches/databases by user permission level.

DoltLab customer has a use-case where different teams/permission levels of people collaborate across various databases, however, only a small number of teams/people are permitted to make schema changes to these databases.

In DoltLab/DoltHub's current permissions model, there is no way to prevent users from making/proposing changes to schema, while allowing changes to the data.

Additionally, the teams/people who review and approve pull requests that _should_ only be adding data, may also not be authorized to changed the schema.

Existing branch permissions do not help with this use-case either.