Set CACHEOPS_DEGRADE_ON_FAILURE to True

This commit is a follow-up to Incident 152 (and 95).

Currently, the app fails with HTTP 5xx errors if the associated
Redis cache is unreachable or cannot be used.

This commit sets the `CACHEOPS_DEGRADE_ON_FAILURE` environment
variable to be `True`. This env var instructs django-cacheops
to wrap Redis calls in a try/except block and to handle
connection errors and timeout errors gracefully, by writing
a warning to the logs, rather than letting the relevant
exception propagate up the stack.

See:
    https://github.com/Suor/django-cacheops#setup
    https://github.com/Suor/django-cacheops/blob/master/cacheops/redis.py#L12-L22

app/config/settings/base.py

@@ -369,6 +369,7 @@
 
 CACHEOPS_ENABLED = True
 CACHEOPS_LRU = True
+CACHEOPS_DEGRADE_ON_FAILURE = True
 
 CACHEOPS_REDIS = {
     "host": REDIS_HOST,