feat: Disable NR for edxapp and gate it on new edxapp-specific variable

[timmc-edx]

Introduces EDXAPP_NEWRELIC_ENABLE and sets it to false so edxapp is no longer drawing from the common COMMON_ENABLE_NEWRELIC_APP variable.

This is now possible thanks to fixes in ddtrace 2.14.2.

See edx/edx-arch-experiments#692

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.