Simplify to whitelist/blacklist model

- Rewrite merge_blocklists.py to sync a single blacklist from upstream
  and subtract the locally-maintained whitelist
- Replace whitelist contents with subtitle + webm seed
- Remove blacklist_permissive, whitelist_with_subtitles, and all
  .prev files that are no longer needed
- Rewrite README to reflect the two-file model and link to wiki

whitelist

@@ -1,3 +1,8 @@
+*.ass
 *.avi
 *.mkv
 *.mp4
+*.srt
+*.ssa
+*.sub
+*.webm