verify publishers on POST adSlot

[Ivshti]

• call verifyPublisher and save the result every time a new adSlot is created; first check if the record exists - if it does, then it's an error (can do that with a single insert)
• a convenience method that takes input, checks blacklist, and saves the successful result or returns issues: an array of strings with problems that the publisher must address (e.g. no DNS record); use this by scripts and POST; the existance of duplicates should also be an issue, but we will still save the record
• if there is a verification error, send it back to the user with the message; return this in the form of issues array of err message strings
• in the platform, show the error and explain how to add DNS TXT record: AmbireTech/adex-platform#394