Ensure b64 prefs always have min padding

This relates to an issue with an unknown cause (unable to reproduce on
my end) where the preferences string does not contain the correct amount
of padding on a base64 encoded value. This is mediated by appending
padding to the end of the encoded value, since any extra padding is
removed anyways.

Fixes #987

app/models/config.py

@@ -254,7 +254,8 @@ class Config:
                 key = self._get_fernet_key(self.preferences_key)
 
                 config = Fernet(key).decrypt(
-                    brotli.decompress(urlsafe_b64decode(preferences.encode()))
+                    brotli.decompress(urlsafe_b64decode(
+                        preferences.encode() + b'=='))
                 )
 
                 config = pickle.loads(brotli.decompress(config))
@@ -262,7 +263,8 @@ class Config:
                 config = {}
         elif mode == 'u': # preferences are not encrypted
             config = pickle.loads(
-                brotli.decompress(urlsafe_b64decode(preferences.encode()))
+                brotli.decompress(urlsafe_b64decode(
+                    preferences.encode() + b'=='))
             )
         else: # preferences are incorrectly formatted
             config = {}