PE-23: add alt callback domain

[baracudda]

For the Reviewer

• Code review complete
• Testing Complete
• Quality ORT App Documentation Updated (your name is in the Validator square for this feature)

When this is complete, you should approve the PR via github.

For the Reviewee

Summary

Add an ENV var for an alternate channel callback domain which applies to all orgs.

Release Note

Add an ENV var for an alternate channel callback domain which applies to all orgs.

Breaking Changes

None.

Quality Assurance

You have gathered the following items:

• This PR is tagged with a Release Milestone
• You have a log message clearly identifying when this feature is working successfully
• You have a log message clearly identifying when this feature is failing
• You added a PR against p4-alerting to trigger based on the failure condition above

Given all of the items above, you have updated your Application ORT at the following locations:

• Features and Alerting: Required.
• P4 Alerting: Required.

Testing and Verification

speficially if your channel callback domain is different from the website's normal domain

[andrewkcarter]

I'm missing the where get_brand_domain() is getting called and the value set to Channel.CONFIG_CALLBACK_DOMAIN, but I assume thats a function call made by some other code you didn't need to touch.