Accessibility: add lang of template, add alt image parameter in case the image is important (as a logo), convert h2 into a paragraph.
Configurable: add nginx variables with default values for configuring the view of the nginx server template.

LEARNER-1845

* Initialize INSIGHTS_CLOUDFRONT to null

* Put cloudfront domain under INSIGHTS_CONFIG

* Use INSIGHTS_CLOUDFRONT_DOMAIN in insights config

* Define INSIGHTS_CLOUDFRONT_DOMAIN null by default

* Set CORS_ORIGIN_WHITELIST to INSIGHTS_DOMAIN

* Add CORS header to Insights static files

* Split CORS whitelist into default and extra

* Allow HEAD, GET, OPTIONS in Insights static

* Try setting CORS Origin to $http_origin

* Use if cond to enable/disable CORS instead of map

* Missing space

* Don't initialize $cors

* Back to the old $cors_origin map

* Don't send CORS whitelist to django settings

* Replace tab with spaces

* Remove allow_all var, and update changelog

* Rename CLOUDFRONT to CDN in var name

This playbook will install carbon-cache and graphite-api to provide a
minimalistic starting point for metrics collection.

It currently installs all components from their upstream source (albeit
with defaults that point to tags, and not master/development branches).
It uses systemd, and so is tailored specifically to Ubuntu 16.04.

It currently does not support any of the advanced configuration such as
multiple carbon-cache process + carbon-relay.  It is also lacking many
variables to fully tweak the various configuration files, although
there's support for fully configuring the storage schemas.

There is a small "master" playbook -- edx-monitoring -- for running the
role, and in the future, supporting the culmination of other playbooks
to build out an all-in-one monitoring server: Graphite, Grafana, etc.

ECOM-7207

There's no such page as /embargo, it yields a 404.
/embargo/blocked-message/courseware/embargo/ is the correct URL.

Previously we only blocked exact matches to "next=favicon.ico". Now,
we'll also block "next=/favicon.ico" and any other variations.

This way it'll apply to all locations

adding whitespace to improve post merge readability, comments.

removing file included in two branches

adding missed template

missed file

missed rename

rename the file to make our convention

clarifying comment.

Stupid is as stupid does.

whitespace

This header gets added to requests proxied by nginx to various edx apps.
The New Relic agent, if there is one, reads this header to calculate the
time each request spends between the web server (nginx) and the
application server (django or some other cms).

The specific intention of this commit is to measure the time requests
spend queued at the gunicorn layer.