cmd/golink: use relative links

use relative links throughout rather than hardcoding the go hostname,
which allows things to continue working when other hostnames are used.

Change-Id: I5df0d10214799de9722871cbad99bf9af129941f

tmpl/help.html

@@ -110,7 +110,7 @@ Include a "+" after a link to get information about a link without resolving it:
 </pre>
 
 <p>
-Visit <a href="http://go/.export">go/.export</a> to export all saved links and their metadata in <a href="https://jsonlines.org/">JSON Lines format</a>.
+Visit <a href="/.export">go/.export</a> to export all saved links and their metadata in <a href="https://jsonlines.org/">JSON Lines format</a>.
 This is useful to create data snapshots that can be restored later.
 
 <pre>{{`$ curl go/.export

tmpl/home.html

@@ -23,7 +23,7 @@
       </thead>
       <tbody>
       {{range .Clicks}}
-        <tr class="hover:bg-gray-100 border-b border-gray-200"><td><a class="block p-2 pr-8 hover:text-blue-500 hover:underline" href="http://go/{{.Short}}">go/{{.Short}}</a></td><td class="p-2">{{.NumClicks}}</td></tr>
+        <tr class="hover:bg-gray-100 border-b border-gray-200"><td><a class="block p-2 pr-8 hover:text-blue-500 hover:underline" href="/{{.Short}}">go/{{.Short}}</a></td><td class="p-2">{{.NumClicks}}</td></tr>
       {{end}}
       </tbody>
     </table>

tmpl/success.html

@@ -1,5 +1,5 @@
 {{ define "main" }}
     <h2 class="text-xl font-bold pb-2">Success</h2>
 
-    <p><a class="text-blue-600 hover:underline" href="http://go/{{.Short}}">go/{{.Short}}</a> has been saved.</p>
+    <p><a class="text-blue-600 hover:underline" href="/{{.Short}}">go/{{.Short}}</a> has been saved.</p>
 {{ end }}