Add 'Singles' type alongside 'Collections'

Original idea as outlined by @erquhart

### Idea:

Allow for a 'Singles' content type, designed for one-off, often publicly-published pages such as 'About', 'Contact', etc. Anything that would not be appropriate as a repeatable object template in the way that 'folder' type Collections work in the CMS today.

This allows for the distinction in the 'Content' interface of these different content types. Collections allow for (+) buttons, Singles do not, as they are one-off. This is largely represented in the 'file' Collection type available in the CMS today.


Start by tracing the Content interface and examining how the existing 'file' Collection type is represented in the CMS. Determine where content types are defined and how the interface renders (+) buttons. Done means a distinct Singles type supports one-off pages and does not show collection creation controls.

Written by the indexing model from the issue text.