fix: fix an unlikely JSON injection security vulnerability

[TomOnTime]

Issue

An MX record with the target mx1.example.com"), A("attacker-controlled", "203.0.113.66 will force dnscontrol get-zone --format js to generate a dnsconfig.js file with an extra A record.

Resolution

JSON-quote all fields.

Risk

Low.

• The attacker would have to access your portal... at which point they could just insert the record with their mouse.
• If the attacker is the DNS service provider, they could serve evil records without you knowing.
• Before you are powned.... every dnscontrol preview would highlight the change.
• Before you are powned.... every dnscontrol push would undo their change.
• While you are being powned... after you run dnscontrol get-zone --format js you are forced to edit the output (it isn't perfect, just "a good first draft") and you'd probably notice this situation, especially if you run dnscontrol fmt

Alas, stranger attacks have happened. Therefore, we're fixing this.

CC @cafferata who did the work, I'm just merging the PR.