creating ad slots: try to verify the website, show issues

[Ivshti]

Is your feature request related to a problem? Please describe.
It's related to this: AmbireTech/adex-market#86

When creating an ad slot, we need to run the verification on the market and display issues, if any.

Related market task: AmbireTech/adex-market#86

Describe the solution you'd like
When creating an ad slot, if it fails, show the reason and the way to solve it (DNS TXT + link to the tutorial). Then, allow to re-try once you add the record.

We'll prioritize DNS TXT verification since it's the most accurate/reliable and works around multiple issues of verifying the integration code.

Also, enforce that the website protocol is https:

If successful, show a message "verification is successful" and stop displaying the "verfiication required" banner on the dashboard