Drop 'unsafe-inline' from the Content-Security-Policy (closes #530)

The delete prompts on the webhook page were inline onsubmit handlers.
Each delete form now carries its question in a data-confirm attribute,
which one submit listener in static/js/app.js asks. The three style
attributes became Tailwind classes, with tailwind.css regenerated, and
the x-cloak rule moved to style.css. The policy now sends
script-src 'self' and style-src 'self'.

The browser test now answers a target's delete prompt no and then yes,
and also loads the sign-in page, the webhook list, the settings and
profile pages and the 404 error page under the policy. The README
states the policy as sent.

Model: opus-5-5

templates/htmlheader.html

@@ -4,6 +4,5 @@
 <title>{{block "title" .}}Webhooker{{end}}</title>
 <link rel="stylesheet" href="/s/css/tailwind.css">
 <link rel="stylesheet" href="/s/css/style.css">
-<style>[x-cloak] { display: none !important; }</style>
 {{block "head" .}}{{end}}
 {{end}}