Explicitly set returnUrl params on element edit pages

[brandonkelly]

Description

Currently after saving a full-page element edit form, the browser is redirected to the referrer URL, so long as it doesn’t look like the URL to the same exact element’s edit page.

That can lead to some awkward behaviors, depending on how you get to the element edit page to begin with. For example, if you press “Save and continue editing” or “Save and add another” from an element’s edit page, the subsequent page will redirect back to the initial edit page (see #18679).

This PR drops the automatic redirection based on the referral URL, in favor of explicitly setting a returnUrl param on element edit pages, when accessing the page from an element index.

It also updates the “Save and continue editing”, “Save and add another”, and “Revert content from this revision” actions to maintain the returnUrl param, so the original element index URL will be remembered even as authors interact with those actions.

Related issues

• Redirect edit pages to the previous page on save #16140
• [5.x]: Unexpected dashboard redirects after save when sites share Translation Keys #18404
• redirect to referral URL on element save again + adjustment #18483
• [5.x]: "Save and add another" results in confusing redirect after subsequent save #18679

[i-just]

that's the only issue I found when testing:

• clean installation running branch for this PR
• create a couple of sections (Blog, My Channel); the entry type can contain all the defaults and only the title
• create a custom page called “Pages”
• add a Blog section to it
• go to CP > Pages > Blog; add an entry to it & fully save
• ensure you’re still in CP > Pages > Blog and edit the entry from the previous step
• choose “save as a new entry” alternative action
• press “Create new entry” (or “Delete draft”)
• you're redirected to the Entries > All entries page; you should be redirected to the Pages > Blog

not sure if this is expected behaviour:

• the setup listed above
• create an Entries field
• add that field to the entry type used by “My Channel”
• ensure you have an entry in that channel and that it has an entry from the Blog section selected in the Entries field
• go to CP > Entries > My Channel and edit the entry from the previous step
• double-click or use the actions menu to edit the entry listed in the Entries field, then open it in a new tab
• press “Save”
• you’re redirected to the Entries > My Channel index page

I get why this is happening, but I’m not sure if this is expected from the user’s POV.