CLI command for moving all nodes+files to another project

`manage.py operations merge_project src_url dst_url` moves all nodes and
files from the project with `src_url` to the project with `dst_url`.
This also moves soft-deleted files/nodes, as it ignores the _deleted
field. The actual files on the storage backend are copied rather than
moved.

Note that this may invalidate the nodes, as their node type definition
may differ between projects. Since we use direct MongoDB queries the
nodes are moved to the new project anyway. This allows for a
move-first-then-fix approach).

tests/test_api/test_project_merging.py

@@ -0,0 +1,75 @@
+import pathlib
+from unittest import mock
+
+from pillar.tests import AbstractPillarTest
+
+from bson import ObjectId
+
+
+class ProjectMergerTest(AbstractPillarTest):
+    def setUp(self, **kwargs):
+        super().setUp(**kwargs)
+
+        self.pid_from, self.uid_from = self.create_project_with_admin(
+            24 * 'a', project_overrides={'url': 'from-url'})
+        self.pid_to, self.uid_to = self.create_project_with_admin(
+            24 * 'b', project_overrides={'url': 'to-url'})
+        self.create_valid_auth_token(24 * 'a', 'from-token')
+        self.create_valid_auth_token(24 * 'b', 'to-token')
+
+    def test_move_happy(self):
+        import pillar.tests.common_test_data as ctd
+        from pillar.api.file_storage_backends.local import LocalBucket
+
+        fid = self._create_file_with_files()
+        nid = self.create_node({
+            **ctd.EXAMPLE_NODE,
+            'picture': fid,
+            'properties': {'file': fid},
+            'project': self.pid_from,
+        })
+
+        from pillar.api.projects import merging
+
+        with self.app.app_context():
+            merging.merge_project(self.pid_from, self.pid_to)
+
+        db_file = self.get(f'/api/files/{fid}').json()
+        db_node = self.get(f'/api/nodes/{nid}').json()
+
+        self.assertEqual(db_file['project'], str(self.pid_to))
+        self.assertEqual(db_node['project'], str(self.pid_to))
+
+        # Check the old and new locations of the files
+        with self.app.app_context():
+            self._assert_files_exist(LocalBucket(self.pid_to), db_file)
+            self._assert_files_exist(LocalBucket(self.pid_from), db_file)
+
+    def _assert_files_exist(self, bucket, db_file):
+        for var in [db_file] + db_file['variations']:
+            fname = var['file_path']
+            blob = bucket.blob(fname)
+            self.assertTrue(blob.exists(),
+                            f'blob for file {fname} does not exist in bucket {bucket}')
+
+    def _create_file_with_files(self):
+        import io
+        from pillar.api.file_storage_backends.local import LocalBucket
+
+        fid, db_file = self.ensure_file_exists({
+            '_id': ObjectId(f'ffff{20 * "a"}'),
+            'project': self.pid_from,
+            'backend': 'local',
+        })
+
+        # Make sure the files on the filesystem exist.
+        with self.app.app_context():
+            bucket = LocalBucket(db_file['project'])
+            for var in [db_file] + db_file['variations']:
+                fname = var['file_path']
+
+                contents = io.BytesIO(fname.encode())
+                blob = bucket.blob(fname)
+                blob.create_from_file(contents, content_type='text/plain')
+
+        return fid