Remove references to 'whitelabel'

[crweiner]

Fixes #782

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

Per the SendGrid Docs here as well as an upcoming addition to the Docs in a PR I have submitted here, the term "Whitelabel" is no longer being used in SendGrid documentation and has been "deprecated" (see here), even though the API still references it.

This PR alters the references to whitelabeling and replaces them with the following:
Whitelabel = Sender Authentication
Domain Whitelabel = Domain Authentication
IP Whitelabel = Reverse DNS
Link Whitelabel = Link branding

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @crweiner,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX