Improve cache hit rate (i.e. cache 404 pages (/404), etc)

[eabquina]

Cache hit rates for the past 5 days are:
26.91%
89.61%
3.51%
73.77%
70.47%

Ideally, we should be consistently at the 90% level.

[eabquina]

Cache hit rate now is averaging 87%

2020-04-13 | 87.14%
2020-04-14 | 90.30%
2020-04-15 | 91.62%
2020-04-16 | 91.65%
2020-04-17 | 85.83%
2020-04-18 | 87.91%
2020-04-19 | 85.94%
2020-04-20 | 92.09%
2020-04-21 | 74.25%
2020-04-22 | 90.41%

Need to verify why cache-control header is missing, we could set this to have a longer cache time (1 week or more)

< HTTP/2 200
< content-security-policy: script-src-elem 'self' 'unsafe-inline' https://www.google.com https://js-agent.newrelic.com https://bam.nr-data.net https://www.gstatic.com https://cdn.datatables.net; script-src 'unsafe-inline' 'unsafe-eval' https://www.google.com https://fonts.googleapis.com https://cdnjs.cloudflare.com https://www.gstatic.com https://js-agent.newrelic.com/
< content-type: text/html; charset=UTF-8
< feature-policy: geolocation "none" ; camera "none"
< link: <https://live-dctx1.pantheonsite.io/wp-json/>; rel="https://api.w.org/"
< link: <https://live-dctx1.pantheonsite.io/>; rel=shortlink
< referrer-policy: no-referrer-when-downgrade
< server: nginx
< strict-transport-security: max-age=31622400
< surrogate-key-raw: front post-7 post-user-1 single
< x-content-type-options: nosniff
< x-frame-options: SAMEORIGIN
< x-pantheon-styx-hostname: styx-fe2-b-b94bb8456-s6s75
< x-styx-req-id: 7246c153-8537-11ea-b49d-faa811817066
< xx-xss-protection: 1; mode=block
< date: Thu, 23 Apr 2020 07:52:59 GMT
< x-served-by: cache-mdw17359-MDW, cache-hkg17930-HKG
< x-cache: MISS, MISS
< x-cache-hits: 0, 0
< x-timer: S1587628377.424742,VS0,VE1659
< vary: Accept-Encoding, Cookie, Cookie
< policy-doc-cache: HIT-CLUSTER
< policy-doc-surrogate-key: live-dctx1.pantheonsite.io
< pcontext-pdocclustering: on
< x-robots-tag: noindex
< age: 1
< accept-ranges: bytes
< via: 1.1 varnish
< content-length: 3577067