#2770: Add validation for httpBypassFqdn as hostname in Traffic Ops

[qtweng]

This PR fixes #2770. A DNS hostname validation is added to httpBypassFqdn in Traffic Ops.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Attempt to reproduce the original issue.

If this is a bugfix, which Traffic Control versions contained the bug?

• 5.1
• 6.0

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[zrhoffman]

In order for CI to pass, #6330 needs to be rebased onto the apache:master branch now that the CI fix in #6329 is merged:

git remote add apache git@github.com:apache/trafficcontrol.git
git fetch apache
git rebase apache
git push your-fork your-branch --force

[rawlinp]

I'm just going to merge as-is since the CI failures are unrelated. I manually tested this validation, and it appears to work fine. Thanks @qtweng for your first contribution!