Expose existing Template Parts in the Inserter

[jameskoster]

To insert an existing template part you must endure the following flow:

1. Insert Template Part block
2. Click "Choose existing"
3. Find the template part

We should consider exposing any existing template part in the Inspector. IE if a template part named "Header" exists, I should see that as a block when I search the Inserter (or the quick inserter) for "Header".

A new section in the Inserter may be required to contain these.

[jameskoster]

This one seems fairly straight-forward. Once a template part is published, it would appear in the "Theme" section of the inserter:

And in the quick inserter when there is an appropriate search query: