Cannot insert file link when Collab Editing Off

A link to a file such as file:///path/to/file is removed when saving the page.

Steps to reproduce

1. Edit a page
2. Open the link dialog and insert a link like file:///path/to/file
3. Save the page
4. Inspect the link and notice there is no href attribute.
5. Edit the page and notice you cannot open the link.

If you edit a page and open the link dialog and insert the link, then click edit on the link again before saving the page, notice that some slashes are missing. The link has been converted to file:/path/to/file

File links don't work when Collab Editing is On. There is a separate issue to address that: https://jira.atlassian.com/browse/CONFSERVER-46039