Support for curated directories

[mbiang]

As a Bonsai administrator, I would like to create landing pages of specific collections of assets. To do this, I would like to provide the following info:

1. Page Title
2. The short name for the slug for the page (ie, 'database-assets')
3. A selection of assets that should be displayed on this page. To select assets, I am willing to navigate to the asset page, and select the collections that this asset should belong to from a selection menu.

This collection should then be visible to public users under the /collections namespace using the page slug provide, and should display the selection of assets available in that collection.

If a user navigations to the /collections page (index), they should see a list of all the page collections.

[mbiang]

@pzupan Some feedback on this feature:

• Each collection should have it's own landing page, for example /collections/database-plugins.
• Admins should be able to delete a collection from the "Collections List" area.