Set expiration dates on reports

[La0]

We could use the navigation session pattern to keep only recently requested reports in redis database.

We would only keep the history points permanently

[La0]

Ideas from #451:

• setup maxmemory.policy
• expire old specific suite reports (keep all:all)

[La0]

I setup the policy on testing:

$ heroku redis:maxmemory -a code-coverage-testing --policy volatile-lru
Maxmemory policy for redis-fitted-26295 (REDIS_URL) set to volatile-lru.
volatile-lru evict keys trying to remove the less recently used keys first, but only those that have an expiry set.

I'm doing the patch to set the expiry.

[La0]

Rule is now also applied in prod. I'm pushing the patch on testing.