CLI: Move alembic imports inside the calling function

Reduces execution time of "manage.py" a bit.

An import of alembic.config takes a relatively long time and
"manage.py" by itself is slow.

ogn/commands/database.py

@@ -1,6 +1,3 @@
-from alembic.config import Config
-from alembic import command
-
 from ogn.commands.dbutils import engine, session
 from ogn.model import Base, AddressOrigin
 from ogn.utils import get_ddb
@@ -14,6 +11,9 @@ manager = Manager()
 def init():
     """Initialize the database."""
 
+    from alembic.config import Config
+    from alembic import command
+
     Base.metadata.create_all(engine)
     alembic_cfg = Config("alembic.ini")
     command.stamp(alembic_cfg, "head")