Release groups in whitelist are not kept after adding a new show

[hergonoway]

Describe the bug
Whatever release group I add in whitelist while adding a new show, there're not kept once the show is added.

To Reproduce

1. add a new show
2. add 1 or more group in whitelist then click "add show"
3. go to the detail page of the show, those groups don't appear, go edit the show, those groups are not in the whitelist box

Expected behavior
whitelisted release groups should be kept after adding the show

Medusa (please complete the following information):
Branch: master
Commit: 918cfe7
Version: 0.5.2
Python Version: | 3.8.5 (default, Jul 20 2020, 23:11:29) [GCC 9.3.0]
SSL Version: | OpenSSL 1.1.1i 8 Dec 2020
OS: | Linux-4.4.59+-x86_64-with

[medariox]

Fixed with #9047