why:
* this gets loaded on every page and the HTML
isn't currently cached at all
* cache for a day to start
closes INTEROP-8276
flag=lti_platform_storage
test plan:
* enable the lti_platform_storage flag
* open the browser network tools and filter by
post_message_forwarding
* load and reload a canvas page a couple of times
* this page should be cached after the first time
* check the Cache-Control response header
* it should have 86400 in it
* bonus:
* open a rails console and run
`Setting.set('post_message_forwarding_ttl', 2.days.seconds.to_s)`
* restart your canvas
* disable the network cache briefly to request the page again
* enable cache again
* check the Cache-Control header on the page
* it should have 172800 in it now
Change-Id: I6f9387a92ba56a1825284abb0ead3369e4c83e90
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/330965
Reviewed-by: Evan Battaglia <ebattaglia@instructure.com>
QA-Review: Evan Battaglia <ebattaglia@instructure.com>
Product-Review: Xander Moffatt <xmoffatt@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
735cbddf49