Create a template loader for SQL templates.

This will automatically find the correct version of a template for the server version, and allows us to remove templates that were previously duplicated for different server versions.

Patch by George & Tira at Pivotal. Review by me and Murtuza from EDB.

Discussion: https://www.postgresql.org/message-id/flat/CAHowoHaU9_pkCt%2B1g8dpY3hsXXZmsJZiJH-3-_Hd%2BC1MxiGhtA%40mail.gmail.com#CAHowoHaU9_pkCt+1g8dpY3hsXXZmsJZiJH-3-_Hd+C1MxiGhtA@mail.gmail.com

web/pgadmin/utils/tests/templates/some_feature/sql/9.1_plus/some_action.sql

@@ -0,0 +1,2 @@
+Some SQL
+some more stuff on a new line

web/pgadmin/utils/tests/templates/some_feature/sql/9.2_plus/some_action.sql

@@ -0,0 +1 @@
+Some 9.2 SQL

web/pgadmin/utils/tests/test_versioned_template_loader.py

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+import os
+
+from flask import Flask
+from jinja2 import FileSystemLoader
+
+from pgadmin import VersionedTemplateLoader
+from pgadmin.utils.route import BaseTestGenerator
+
+
+class TestVersionedTemplateLoader(BaseTestGenerator):
+    def setUp(self):
+        self.loader = VersionedTemplateLoader(FakeApp())
+
+    def runTest(self):
+        self.test_get_source_returns_a_template()
+        self.test_get_source_when_the_version_is_9_1_returns_9_1_template()
+        self.test_get_source_when_the_version_is_9_3_and_there_are_templates_for_9_2_and_9_1_returns_9_2_template()
+
+    def test_get_source_returns_a_template(self):
+        expected_content = "Some SQL" \
+                           "\nsome more stuff on a new line\n"
+
+        content, filename, up_to_dateness = self.loader.get_source(None, "some_feature/sql/9.1_plus/some_action.sql")
+
+        self.assertEqual(expected_content, content)
+        self.assertIn("some_feature/sql/9.1_plus/some_action.sql", filename)
+
+    def test_get_source_when_the_version_is_9_1_returns_9_1_template(self):
+        expected_content = "Some SQL" \
+                           "\nsome more stuff on a new line\n"
+
+        content, filename, up_to_dateness = self.loader.get_source(None, "some_feature/sql/#90100#/some_action.sql")
+
+        self.assertEqual(expected_content, content)
+        self.assertIn("some_feature/sql/9.1_plus/some_action.sql", filename)
+
+    def test_get_source_when_the_version_is_9_3_and_there_are_templates_for_9_2_and_9_1_returns_9_2_template(self):
+        content, filename, up_to_dateness = self.loader.get_source(None, "some_feature/sql/#90300#/some_action.sql")
+
+        self.assertEqual("Some 9.2 SQL", content)
+        self.assertIn("some_feature/sql/9.2_plus/some_action.sql", filename)
+
+
+class FakeApp(Flask):
+    def __init__(self):
+        super(FakeApp, self).__init__("")
+        self.jinja_loader = FileSystemLoader(os.path.dirname(os.path.realpath(__file__)) + "/templates")