Fix <a> link escaping in web template (#5019)

# What this PR does
Removed escaping of html in web template to fix issue where <a> links
were not being detected correctly when post processing the web template.

## Which issue(s) this PR closes

https://github.com/grafana/support-escalations/issues/12412

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## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/alerts/incident_appearance/templaters/web_templater.py

@@ -15,7 +15,7 @@ class AlertWebTemplater(AlertTemplater):
         if templated_alert.title:
             templated_alert.title = escape_html(self._slack_format_for_web(templated_alert.title))
         if templated_alert.message:
-            message = escape_html(self._slack_format_for_web(templated_alert.message))
+            message = self._slack_format_for_web(templated_alert.message)
             link_matches = re.findall(url_re, message)
             for idx, link in enumerate(link_matches):
                 substitution = f"oncallsubstitutedlink{idx}marker"

engine/apps/alerts/tests/test_alert_group_renderer.py

@@ -55,6 +55,50 @@ def test_render_web_alert_links(
     )
 
 
+@pytest.mark.parametrize(
+    "message, expected_result",
+    [
+        (
+            '<a href="https://www.google.com">google</a>',
+            '<p><a href="https://www.google.com" rel="nofollow noopener" target="_blank">google</a> </p>',
+        ),
+        (
+            '<a href="http://www.google.com">google</a>',
+            '<p><a href="http://www.google.com" rel="nofollow noopener" target="_blank">google</a> </p>',
+        ),
+        (
+            '<a href="//www.google.com">google</a>',
+            '<p><a href="//www.google.com" rel="nofollow noopener" target="_blank">google</a> </p>',
+        ),
+        ("http://www.google.com/", '<p><a href="http://www.google.com/">http://www.google.com/</a> </p>'),
+        (
+            "[Hello](http://www.google.com)",
+            '<p><a href="http://www.google.com" rel="nofollow noopener" target="_blank">Hello</a> </p>',
+        ),
+    ],
+)
+@pytest.mark.django_db
+def test_render_web_postformat_html_a_links(
+    make_organization_and_user_with_slack_identities,
+    make_alert_receive_channel,
+    make_alert_group,
+    make_alert,
+    message,
+    expected_result,
+):
+    organization, _, _, _ = make_organization_and_user_with_slack_identities()
+    alert_receive_channel = make_alert_receive_channel(
+        organization,
+    )
+    alert_group = make_alert_group(alert_receive_channel)
+
+    alert = make_alert(alert_group=alert_group, raw_request_data={"message": message})
+
+    templater = AlertWebTemplater(alert)
+    templated_alert = templater.render()
+    assert templated_alert.message == expected_result
+
+
 @pytest.mark.django_db
 def test_getattr_template(
     make_organization_and_user_with_slack_identities,