Rich text page links use admin display title instead of regular title

Issue Summary

When inserting a link to a page from the rich text editor, the anchor text that is pre-populated (if you haven’t selected some text to anchor) is generated from the admin page title (Page.get_admin_display_title). This is not expected behaviour, because this is not the public page title that is visible on the published page.

Steps to Reproduce

1. Override Page.get_admin_display_title on a page model - for example to include some additional context about the page such as when it was created. This should only be visible when viewing pages in the admin.
2. In a rich text field, add a link to this page. The auto-populated anchor text will use the admin display title instead of just the title field value.

Technical details

• Python version: 3.6
• Django version: 2.0
• Wagtail version: master

[gasman]

@chosak Sorry, I should have been more specific there… I think the use of the draft title as the default link text for generated links in rich text is an oversight. Displaying the draft title to editors within listings etc is appropriate (and so no changes to get_admin_display_title are necessary), but that’s where it should end - the draft title shouldn’t find its way into other pages’ content.

(This does mean that in this one specific case, where a title has been edited in draft, there’ll be a slightly jarring effect where the editor sees one title in the listing, clicks on it, and ends up with a different title as the link text. I think that behaviour is justified, though.)

[gasman]

Fixed in #5491