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).

pillar/api/projects/merging.py

@@ -0,0 +1,44 @@
+"""Code for merging projects."""
+import logging
+
+from bson import ObjectId
+
+from pillar import current_app
+from pillar.api.file_storage.moving import move_to_bucket
+from pillar.api.utils import random_etag, utcnow
+
+log = logging.getLogger(__name__)
+
+
+def merge_project(pid_from: ObjectId, pid_to: ObjectId):
+    """Move nodes and files from one project to another.
+
+    Note that this may invalidate the nodes, as their node type definition
+    may differ between projects.
+    """
+    log.info('Moving project contents from %s to %s', pid_from, pid_to)
+    assert isinstance(pid_from, ObjectId)
+    assert isinstance(pid_to, ObjectId)
+
+    files_coll = current_app.db('files')
+    nodes_coll = current_app.db('nodes')
+
+    # Move the files first. Since this requires API calls to an external
+    # service, this is more likely to go wrong than moving the nodes.
+    to_move = files_coll.find({'project': pid_from}, projection={'_id': 1})
+    log.info('Moving %d files to project %s', to_move.count(), pid_to)
+    for file_doc in to_move:
+        fid = file_doc['_id']
+        log.debug('moving file %s to project %s', fid, pid_to)
+        move_to_bucket(fid, pid_to)
+
+    # Mass-move the nodes.
+    etag = random_etag()
+    result = nodes_coll.update_many(
+        {'project': pid_from},
+        {'$set': {'project': pid_to,
+                  '_etag': etag,
+                  '_updated': utcnow(),
+                  }}
+    )
+    log.info('Moved %d nodes to project %s', result.modified_count, pid_to)