Add 'edit' option to custom DNS and CNAME

[martinbutt]

Signed-off-by: Martin Butt git@martinbutt.com

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• What does this PR aim to accomplish?:

This PR adds an 'edit' action to the custom DNS/CNAME commands in the CLI. This allows an atomic operation to edit existing entries without knowing the previous target IP address/hostname. Without this change there are three actions required to edit an entry. One to look up the current target, one to delete that entry, and one to add the new one. This simplifies that process.

• How does this PR accomplish the above?:

This PR adds two new functions to advanced/Scripts/webpage.sh: EditCustomDNSAddress and EditCustomCNAMERecord that delete the entry in the file and add a new one. These are different from simply deleting and adding and entry through the current CLI, as these new functions require no knowledge of the previous entry.

• What documentation changes (if any) are needed to support this PR?:

I can't find any existing documentation on the current CLI for custom DNS and CNAME entries. If someone points me to any, I'll be happy to update it.

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