feat(downloads): a clip or an edit can be a GIF, shared as a picture

A clip and an editor range can come out as a silent GIF (long side
320/480/640, 12 fps, at most 30 s, one palette per GIF); one over 30 MB
is not kept. A shared picture is served inline under the preview's
sandbox, and a GIF's link card is the GIF itself up to 15 MB, the still
for Facebook's crawler.

backend/tests/test_download_errors.py

@@ -75,6 +75,8 @@ def test_our_own_worker_refusals_classify_as_themselves():
     assert classify(dlerrors.CLIP_TOO_BIG_MESSAGE) == "quota_exceeded"
     assert classify(dlerrors.CONVERTED_TOO_BIG_MESSAGE) == "quota_exceeded"
     assert classify(dlerrors.DOWNLOAD_TOO_BIG_MESSAGE) == "quota_exceeded"
+    # A GIF over its own cap is not the storage: its own category, its own sentence.
+    assert classify(dlerrors.GIF_TOO_BIG_MESSAGE) == "gif_too_big"
 
 
 def test_the_redacted_network_text_classifies_back_to_network():