Possibility to add a redis password #1283

Describe the bug
When using redis as cache/session driver, one might need to add a password for the redis connection, there is no variable in the config or any mention in the db file

Steps To Reproduce
Steps to reproduce the behavior:

1. Change drivers to redis
2. Look for the place to put the password

Expected behavior
Either have a REDIS_PASSWORD variable or use something like REDIS_SERVERS=:${REDIS_PASSWORD}@${REDIS_HOST}:${REDIS_PORT}

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): v0.25.1
• PHP Version: 7.2
• Hosting Method (Nginx/Apache/Docker): apache in a docker container behind nginx reverse proxy

Additional context
Hope I didn't miss something