Add validator for TenantMixin.schema_name, check for 'pg_' prefix

From the PostgreSQL doc (http://www.postgresql.org/docs/9.1/static/ddl-schemas.html):
```Schema names beginning with pg_ are reserved for system purposes and cannot be created by users```
So we should check for it.

Also I've refactored checks for valid identifiers/schema names and have added a validator to the `TenantMixin.schema_name` field which helps to validate the attribute in case there is a form with the corresponding field.