Add Request.internal_comments

Using a one to one relationship to avoid the migration hell
that we're temporarily stuck in.

atst/models/request_internal_comment.py

@@ -0,0 +1,16 @@
+from sqlalchemy import Column, BigInteger, String, ForeignKey
+from sqlalchemy.orm import relationship
+
+from atst.models import Base
+
+
+class RequestInternalComment(Base):
+    __tablename__ = "request_internal_comments"
+
+    id = Column(BigInteger, primary_key=True)
+    text = Column(String())
+
+    user_id = Column(ForeignKey("users.id"), nullable=False)
+    user = relationship("User")
+
+    request_id = Column(ForeignKey("requests.id", ondelete="CASCADE"))