Update Qt 5.15 session warning again

Also don't show it for new users - this doesn't really help much if
someone just started using qutebrowser.

See #5359

qutebrowser/app.py

@@ -372,6 +372,12 @@ def _open_special_pages(args):
          'qute://warning/sessions'),
     ]
 
+    if 'quickstart-done' not in general_sect:
+        # New users aren't going to be affected by the Qt 5.15 session change much, as
+        # they aren't used to qutebrowser saving the full back/forward history in
+        # sessions.
+        general_sect['session-warning-shown'] = '1'
+
     for state, condition, url in pages:
         if general_sect.get(state) != '1' and condition:
             tabbed_browser.tabopen(QUrl(url), background=False)

qutebrowser/html/warning-sessions.html

@@ -9,7 +9,7 @@ <h1>{{ title }}</h1>
 
 <p>Since Qt doesn't provide an API to load the history of a tab, qutebrowser relies on a reverse-engineered binary serialization format to load tab history from session files. With Qt 5.15, unfortunately that format changed (due to the underlying Chromium upgrade), in a way which makes it impossible for qutebrowser to load tab history from existing session data.</p>
 
-<p>At the time of writing (October 2020), a new session format which stores part of the needed binary data in saved sessions is <a href="https://github.com/qutebrowser/qutebrowser/issues/5359">in development</a> and is expected to be released with qutebrowser v2.0.0 (planned to be released at the end of the year or early 2021).</p>
+<p>At the time of writing (January 2021), a new session format which stores part of the needed binary data in saved sessions is <a href="https://github.com/qutebrowser/qutebrowser/issues/5359">in development</a>. However, it unfortunately wasn't ready in time for qutebrowser v2.0.0, as it's a rather big refactoring. It's currently expected to be released as part of qutebrowser v2.1.0.</p>
 
 <p>As a stop-gap measure:</p>