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.

src/templates/nodes/attachments/file_image.pug

@@ -1,11 +1,11 @@
-| {% if 'link' in attachment and attachment['link'] != 'none' %}
-| {% if attachment['link'] == 'self' %}
+| {% if 'link' in tag_args and tag_args['link'] != 'none' %}
+|     {% if tag_args['link'] == 'self' %}
 a(href="{{ vars['l'].link }}")
 	img(src="{{ vars['l'].link }}", alt="{{ file.filename }}")
-| {% elif attachment['link'] == 'custom' %}
-a(href="{{ attachment['link_custom'] }}")
+|     {% else %}
+a(href="{{ tag_args['link'] }}", target="_blank")
 	img(src="{{ vars['l'].link }}", alt="{{ file.filename }}")
-| {% endif %}
+|     {% endif %}
 | {% else %}
 img(src="{{ vars['l'].link }}", alt="{{ file.filename }}")
 | {% endif %}