Make the invite copyable and widen what feedback invites

Three fixes from using the admin panel and the site:

- The invite link was an anchor, but an invite is something to send, not to
  follow — clicking it opened the join form in the admin's own browser. It
  is now the URL beside a Kopioi button. The handler reads the text out of
  the sibling element rather than interpolating the URL into JS, so there is
  nothing to escape, and where the clipboard API is missing (it needs a
  secure context, which the documented SSH tunnel to localhost provides) it
  selects the text instead of leaving a button that does nothing.
- "Ilmoita ongelmasta" framed the feedback form as a bug tracker when it is
  meant to take ideas and general feedback too. The footer now asks
  "Ongelmia? Ideoita? Palautetta?", and the page it leads to answers all
  three: the ingress covers ideas explicitly and the placeholder suggests a
  feature rather than a fault.
- "Kuuntele YouTubessa" opens in a new tab. Leaving the page mid-review
  would lose whatever is already typed into the review form.

templates/layout.html

@@ -62,7 +62,7 @@
   <footer class="sitefooter">
     {{if .Member}}
       <!-- The server already knows where they were, so the path travels in the link — no JS. -->
-      <a href="/report?from={{.Path}}">Ilmoita ongelmasta</a> ·
+      <a href="/report?from={{.Path}}">Ongelmia? Ideoita? Palautetta?</a> ·
     {{end}}
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