[ADD] donate external link handler

donate/wagtail_hooks.py

+from django.utils.html import escape
+from wagtail.core import hooks
+from wagtail.core.rich_text import LinkHandler
+
+
+class ExternalLinkHandler(LinkHandler):
+    identifier = "external"
+
+    @classmethod
+    def expand_db_attributes(cls, attrs):
+        href = attrs["href"]
+        return '<a href="%s" target="_blank" rel="noopener nofollower">' % escape(href)
+
+
+@hooks.register("register_rich_text_features")
+def register_external_link(features):
+    features.register_link_type(ExternalLinkHandler)