fix: prevent slide upload by participants after session (#10582)

[jimfenton]

Restricts slide submission by other than those that can manage slides after the session has ended.

Does not include tests for this, and breaks two existing tests:

======================================================================
FAIL: test_propose_session_slides (ietf.meeting.tests_views.MaterialsTests.test_propose_session_slides)

Traceback (most recent call last):
File "/workspace/ietf/meeting/tests_views.py", line 7077, in test_propose_session_slides
self.assertTrue(q('.proposeslides'))
AssertionError: [] is not true

======================================================================
FAIL: test_submit_and_approve_multiple_versions (ietf.meeting.tests_views.MaterialsTests.test_submit_and_approve_multiple_versions)

Traceback (most recent call last):
File "/home/dev/.local/lib/python3.12/site-packages/django/test/utils.py", line 461, in inner
return func(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/unittest/mock.py", line 1396, in patched
return func(*newargs, **newkeywargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/workspace/ietf/meeting/tests_views.py", line 7271, in test_submit_and_approve_multiple_versions
self.assertEqual(r.status_code, 302)
AssertionError: 403 != 302

Apparently this was because the sessions created for testing did not specify a time. I'm surprised this didn't fail before, because the submission would probably have violated the grace time after which Secretariat action is needed. I tried but didn't manage to succeed in fixing these test cases by adding a session time to the SessionFactory calls.

[jimfenton]

I found another "Upload slides" button that isn't covered by this fix yet (materials/views.py line 1937). Will add.