Merge branch 'add-a-version-number-to-exports' into 'master'

Add a version number to exports

Closes #829

See merge request !1462

arkindex/documents/export/structure.sql

 PRAGMA foreign_keys = ON;
 
+CREATE TABLE export_version AS SELECT 2 AS version;
+
 CREATE TABLE image_server (
     id VARCHAR(37) NOT NULL,
     display_name VARCHAR(250) NOT NULL,

arkindex/documents/tests/tasks/test_export.py

@@ -27,6 +27,7 @@ from arkindex.project.tests import FixtureTestCase
 from ponos.models import Artifact
 
 TABLE_NAMES = {
+    'export_version',
     'classification',
     'element',
     'element_path',
@@ -143,6 +144,10 @@ class TestExport(FixtureTestCase):
             [(name, ) for name in TABLE_NAMES],
         )
 
+        self.assertCountEqual(
+            db.execute("SELECT version FROM export_version").fetchall(), [(2, )]
+        )
+
         self.assertCountEqual(
             db.execute('SELECT id, display_name, url, max_width, max_height FROM image_server').fetchall(),
             [