Support the rel attribute on ModelAdmin buttons

Currently it is possible to pass the target attribute to a button
created using ModelAdmin's ButtonHelper framework. This allows you to
generate button links like <a ... target="_blank">.

For example, if adding a new button and modeling it after the existing
edit_button code [0], you can add {'target': "_blank"} to the returned
dict and it'll get passed to the template when the button is rendered.

To be consistent with PR 4844, and to be consistent with what seems to
be the best practice ([1], [2]), we should also support passing the rel
attribute, which would allow for creation of button links like <a ...
target="_blank" rel="noopener noreferrer">.

[0] 5e2f50403b/wagtail/contrib/modeladmin/helpers/button.py (L61-L73)
[1] https://developers.google.com/web/tools/lighthouse/audits/noopener
[2] https://www.jitbit.com/alexblog/256-targetblank---the-most-underestimated-vulnerability-ever/

CHANGELOG.txt

@@ -10,6 +10,7 @@ Changelog
  * Fix: Re-add 'Close Explorer' button on mobile viewports (Sævar Öfjörð Magnússon)
  * Fix: Add a more descriptive label to Password reset link for screen reader users (Casper Timmers, Martin Coote)
  * Fix: Improve Wagtail logo contrast by adding a background (Brian Edelman, Simon Evans, Ben Enright)
+ * Support the rel attribute on custom ModelAdmin buttons (Andy Chosak)
 
 
 2.8 (xx.xx.xxxx) - IN DEVELOPMENT

docs/releases/2.9.rst

@@ -15,6 +15,7 @@ Other features
 ~~~~~~~~~~~~~~
 
  * Reduced contrast of rich text toolbar (Jack Paine)
+ * Support the rel attribute on custom ModelAdmin buttons (Andy Chosak)
 
 
 Bug fixes

wagtail/contrib/modeladmin/templates/modeladmin/includes/button.html

@@ -1 +1 @@
-<a{% if button.url %} href="{{ button.url }}"{% endif %} class="{{ button.classname }}" title="{{ button.title }}"{% if button.target %} target="{{ button.target }}"{% endif %}>{{ button.label }}</a>
+<a{% if button.url %} href="{{ button.url }}"{% endif %} class="{{ button.classname }}" title="{{ button.title }}"{% if button.target %} target="{{ button.target }}"{% endif %}{% if button.rel %} rel="{{ button.rel }}"{% endif %}>{{ button.label }}</a>