Handling of operational webhooks server name

[boothj5]

After having an issue running local operational webhooks (see the following pull request #1137) it looks as though handling the server url could do with a little more validation.

Comments from @tasn in Slack chat:

"we should probably have validation (or rather URL sanitization?) to fix this class of issues going forward."

"we should have also checked if it's a 404 because the org is not found or if it's just a bad path, so we should have improved that too."

The following discussion also has some more information: #1125

[svix-jplatte]

We now do basic validation of the URL, and trailing slashes are automatically trimmed too, as of #1887.