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

archivebox/cli/tests_piping.py

@@ -44,7 +44,7 @@ TEST_CONFIG = {
     'SAVE_READABILITY': 'False',
     'SAVE_MERCURY': 'False',
     'SAVE_GIT': 'False',
-    'SAVE_MEDIA': 'False',
+    'SAVE_YTDLP': 'False',
     'SAVE_HEADERS': 'False',
     'USE_CURL': 'False',
     'USE_WGET': 'False',