rename media plugin to ytdlp with backwards-compatible aliases

- Rename archivebox/plugins/media/ → archivebox/plugins/ytdlp/
- Rename hook script on_Snapshot__63_media.bg.py → on_Snapshot__63_ytdlp.bg.py
- Update config.json: YTDLP_* as primary keys, MEDIA_* as x-aliases
- Update templates CSS classes: media-* → ytdlp-*
- Fix gallerydl bug: remove incorrect dependency on media plugin output
- Update all codebase references to use YTDLP_* and SAVE_YTDLP
- Add backwards compatibility test for MEDIA_ENABLED alias

tests/fixtures.py

@@ -23,7 +23,7 @@ def disable_extractors_dict():
         "SAVE_DOM": "false",
         "SAVE_HEADERS": "false",
         "USE_GIT": "false",
-        "SAVE_MEDIA": "false",
+        "SAVE_YTDLP": "false",
         "SAVE_ARCHIVEDOTORG": "false",
         "SAVE_TITLE": "false",
         "SAVE_FAVICON": "false",