Change Flask-Session redis_store config_prefix

The "REDIS_URL" variable is hard-coded in RQ Dashboard.

REDIS_URL was used by redis_store that is passed to Flask Session.
The url variable used by Flask-Redis can be changed by setting the
config_prefix.

1. Rename REDIS_URL to SESSION_REDIS_URL
2. Rename redis_store to session_redis_store
3. Use "SESSION_REDIS" config_prefix to initialize session_redis_store

This makes it clear that this Redis instance is used by Flask-Session.
REDIS_URL will be used for RQ.