`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).
