Use dogpile.cache sha1_mangle_key to mangle cache keys

When using the subtitle hashes as cache keys, sometimes they come
out as filenames of 270 characters.
Not a lot of filesystems out there support filenames with over 250
characters.
This behaviour was reported with ext4 and zfs on both linux and FreeBSD.

The dogpile.cache utils come with a function that returns a hex digest
of the key, limiting the filename to a fixed length of 40 characters.

bazarr/init.py

@@ -11,7 +11,9 @@ from get_args import args
 from logger import configure_logging
 from helper import path_mappings
 
+from dogpile.cache import make_region
 from dogpile.cache.region import register_backend as register_cache_backend
+from dogpile.cache.util import sha1_mangle_key
 import subliminal
 import datetime
 
@@ -109,12 +111,19 @@ if not os.path.exists(os.path.join(args.config_dir, 'db', 'bazarr.db')):
 
 # upgrade database schema
 from database import db_upgrade
+
 db_upgrade()
 
 # Configure dogpile file caching for Subliminal request
 register_cache_backend("subzero.cache.file", "subzero.cache_backends.file", "SZFileBackend")
-subliminal.region.configure('subzero.cache.file', expiration_time=datetime.timedelta(days=30),
-                            arguments={'appname': "sz_cache", 'app_cache_dir': args.config_dir})
+
+subliminal.region = make_region(
+    key_mangler=sha1_mangle_key
+).configure(
+    'subzero.cache.file',
+    expiration_time=datetime.timedelta(days=30),
+    arguments={'appname': "sz_cache", 'app_cache_dir': args.config_dir}
+)
 subliminal.region.backend.sync()
 
 if not os.path.exists(os.path.join(args.config_dir, 'config', 'releases.txt')):