Linkit should recognize Canvas pages as targets

Problem/motivation

Linkit works within Drupal Canvas just fine, but the Linkit matches found don't currently include pages created within Canvas.

Steps to reproduce

1. Create a page in Drupal Canvas, and name it
2. Somewhere else on your site, highlight some text in CKEditor and click to link it (using a text format configured to use Linkit)
3. Observe that the Canvas-created page is not offered as a suggested match

Proposed resolution

Have Linkit recognize Canvas pages as potential matches.