modify schema primary keys to be nullable+defaulted

This fixes SQLAlchemy warnings related to primary keys not
having an auto_increment or nullable.

We've done this by making all foreign primary keys nullable.

In ApiRateLimit's case, we can set a default str to act as
a null, which seems a bit more sensible.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/models/package_keyword.py

@@ -1,4 +1,5 @@
 from sqlalchemy.orm import mapper
+from sqlalchemy.exc import IntegrityError
 
 from aurweb.db import make_relationship
 from aurweb.models.package_base import PackageBase
@@ -10,6 +11,12 @@ class PackageKeyword:
                  PackageBase: PackageBase = None,
                  Keyword: str = None):
         self.PackageBase = PackageBase
+        if not self.PackageBase:
+            raise IntegrityError(
+                statement="Primary key PackageBaseID cannot be null.",
+                orig="PackageKeywords.PackageBaseID",
+                params=("NULL"))
+
         self.Keyword = Keyword