Remove references to "Whitelabel"

[crweiner]

Closes #686

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 #686 and 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.

[codecov]

Codecov Report

Merging #687 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #687   +/-   ##
=======================================
  Coverage   84.77%   84.77%           
=======================================
  Files          35       35           
  Lines        1156     1156           
  Branches      172      172           
=======================================
  Hits          980      980           
  Misses         90       90           
  Partials       86       86

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[misterdorm]

Thanks for this extensive doc update!

[thinkingserious]

Hello @crweiner,

Thanks again for the PR!

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