Improve accessibility of messages in admin.

[Sarah Boyce]

1. The message icons in RTL languages should be on the right

2. The color contrast of the success message (yes icon) fails the AA guidelines in both light and dark mode. The warning message (alert icon) fail the AA guidelines in light mode.

3. In forced colors mode, the message probably needs a border. Note that this can be partially acheived by giving a bottom border to the header breadcrumbs

[Sarah Boyce]

Note that the following screenshot test can be used to generate screenshots for testing

tests/admin_views/tests.py

@@ -135 +135 @@
     UnchangeableObject,
     UndeletableObject,
     UnorderedObject,
+    UserMessenger,
     UserProxy,
     Villain,
     Vodcast,
@@ -6873 +6874 @@
         name_input_value = name_input.get_attribute("value")
         self.assertEqual(name_input_value, "Test section 1")
 
+    @screenshot_cases(["desktop_size", "mobile_size", "rtl", "dark", "high_contrast"])
+    @override_settings(MESSAGE_LEVEL=10)
+    def test_messages(self):
+        from selenium.webdriver.common.by import By
+        from selenium.webdriver.support.ui import Select
+
+        self.admin_login(
+            username="super", password="secret", login_url=reverse("admin:index")
+        )
+        UserMessenger.objects.create()
+        for level in ["warning", "info", "error", "success", "debug"]:
+            self.selenium.get(
+                self.live_server_url + reverse("admin:admin_views_usermessenger_changelist"),
+            )
+            checkbox = self.selenium.find_element(
+                By.CSS_SELECTOR, "tr input.action-select"
+            )
+            checkbox.click()
+            Select(self.selenium.find_element(By.NAME, "action")).select_by_value(
+                f"message_{level}"
+            )
+            self.selenium.find_element(By.XPATH, '//button[text()="Run"]').click()
+            message = self.selenium.find_element(By.CSS_SELECTOR, "ul.messagelist li")
+            self.assertEqual(message.get_attribute("innerText"), f"Test {level}")
+            self.take_screenshot(level)
+