Small bug fixes

- Delete from Releases when deleting artist/album
- Searcher - Size limits not quite working
- Searcher - 1st newznab used even if disabled
- Rutracker search stopped working for me, fixed by updating Beautiful
Soup. Moved bs4 and html5lib to lib and ensured (I think) it’s imported
from the right place

lib/html5lib/trie/_base.py

@@ -0,0 +1,37 @@
+from __future__ import absolute_import, division, unicode_literals
+
+from collections import Mapping
+
+
+class Trie(Mapping):
+    """Abstract base class for tries"""
+
+    def keys(self, prefix=None):
+        keys = super().keys()
+
+        if prefix is None:
+            return set(keys)
+
+        # Python 2.6: no set comprehensions
+        return set([x for x in keys if x.startswith(prefix)])
+
+    def has_keys_with_prefix(self, prefix):
+        for key in self.keys():
+            if key.startswith(prefix):
+                return True
+
+        return False
+
+    def longest_prefix(self, prefix):
+        if prefix in self:
+            return prefix
+
+        for i in range(1, len(prefix) + 1):
+            if prefix[:-i] in self:
+                return prefix[:-i]
+
+        raise KeyError(prefix)
+
+    def longest_prefix_item(self, prefix):
+        lprefix = self.longest_prefix(prefix)
+        return (lprefix, self[lprefix])