Turn matrix.to links into shorter forms

e.g. if someone posts a message containing
"https://matrix.to/#/@foo:example.org", that link will be turned into a
properly colored "@foo:example.org".

src/backend/html_markdown.py

@@ -162,6 +162,8 @@ class HTMLProcessor:
         user_id_regex, room_id_regex, room_alias_regex,
     ]]
 
+    matrix_to_regex = re.compile(r"^https?://matrix.to/#/", re.IGNORECASE)
+
     link_is_matrix_to_regex = re.compile(
         r"https?://matrix.to/#/\S+", re.IGNORECASE,
     )
@@ -526,6 +528,9 @@ class HTMLProcessor:
         if not href or not el.text:
             return el
 
+
+        el.text = self.matrix_to_regex.sub("", el.text or "")
+
         # This must be first, or link will be mistaken by room ID/alias regex
         if self.link_is_message_id_regex.match(href):
             el.attrib["class"]        = "mention message-id-mention"