Render attachments with shortcodes rather than slugs

The attachments should now be rendered using `{attachment slug}` instead
of `@[slug]`. The `link` attribute can be specified in the shortcode
(for attachments that support it), rather than in the attachment itself.

The attachment subdocument is now reduced to `{oid: File ObjectID}`, and
nodes without attachments should NOT have an `attachment` property at
all (previously it would be an empty dict). This makes querying for
nodes with/out attachments easier.

The CLI command `upgrade_attachment_schema` can do dry-run and remove
empty attachments:

- Added --go to actually perform the database changes.
- Remove empty attachments, so that a node either has one or more
  attachments or no attachments sub-document at all.

The CLI command `upgrade_attachment_usage` converts `@[slug]` to
`{attachment slug}`. It also takes into account 'link' and 'link_custom'
fields on the attachment. After conversion those fields are removed from
the attachment itself.

Simplified maintentance CLI commands that iterate over all projects:
I've moved the common approach (either run on one project or all of
them, skipping deleted ones, giving a message upon dry-run, and showing
duration of the command) to a new _db_projects() function. The new
function is now used by two recently-touched CLI commands; more of them
could be migrated to use this.

pillar/api/projects/utils.py

@@ -1,4 +1,5 @@
 import logging
+import typing
 
 from bson import ObjectId
 from werkzeug import exceptions as wz_exceptions
@@ -135,6 +136,14 @@ def get_node_type(project, node_type_name):
                  if nt['name'] == node_type_name), None)
 
 
+def node_type_dict(project: dict) -> typing.Dict[str, dict]:
+    """Return the node types of the project as dictionary.
+
+    The returned dictionary will be keyed by the node type name.
+    """
+    return {nt['name']: nt for nt in project['node_types']}
+
+
 def project_id(project_url: str) -> ObjectId:
     """Returns the object ID, or raises a ValueError when not found."""