Shorten reply text for room list last messages

For rooms in the room list, when the last message displayed under the
room name is a reply, change the
"In reply to @foo:example.org: original message... ⏎" text shown
before the actual reply content to just "↩ foo: ".

This lets the user preview the actual interesting part of the message,
instead of a long "In reply to" that takes all the available width.

src/backend/html_markdown.py

@@ -484,7 +484,15 @@ class HTMLProcessor:
         if el.tag != "mx-reply":
             return el
 
-        el.tail = f" ⏎⏎ {el.tail or ''}"
+        try:
+            user_id = el.find("blockquote").findall("a")[1].text
+            text    = f"↩ {user_id[1:].split(':')[0]}: "  # U+21A9 arrow
+        except (AttributeError, IndexError):
+            text = "↩ "  # U+21A9 arrow
+
+        el.clear()
+        el.tag  = "span"
+        el.text = text
         return el