[Configuration][Migration] Use 'alembic' for migration of the SQLite

based configuration file from one version to another, and also allows us
to have a single path of creating the table instead of creating tables
using SQLAlchemy or hand rolled SQL

This allows us to run the migrations directly in the code, and it will
avoid the error prone version numbering.

Patched by: Sarah McAlear
Revisions: Joao Pedro De Almeida Pereira, George Gelashvili.
Reviewed by: Ashesh Vashi, Murtuza Zabuawala

web/pgadmin/setup/db_version.py

@@ -0,0 +1,28 @@
+##########################################################################
+#
+# pgAdmin 4 - PostgreSQL Tools
+#
+# Copyright (C) 2013 - 2017, The pgAdmin Development Team
+# This software is released under the PostgreSQL Licence
+#
+##########################################################################
+
+from pgadmin.model import Version
+import config
+import sys
+
+
+def get_version():
+    try:
+        version = Version.query.filter_by(name='ConfigDB').first()
+    except Exception:
+        return -1
+
+    if int(version.value) > int(config.SETTINGS_SCHEMA_VERSION):
+        print(u"""
+    The database schema version is {0}, whilst the version required by the \
+    software is {1}.
+    Exiting...""".format(version.value, config.SETTINGS_SCHEMA_VERSION))
+        sys.exit(1)
+
+    return version.value