Insert link dialog and populated relative link list

This commit adds a link dialog that:

- Provides a link field and optional title field in a dialog dropdown for insertion into a document.
- Reads a selected link and populates form values in the link dialog
- Provides the option to specify a `relativeLinks` [in configuration](https://github.com/tristen/documents/blob/master/_prose.yml#L3) that populates a filterable dropdown to insert local relative links.