let admins and users change usernames

server/assets/pages/change_username.tmpl

@@ -0,0 +1,11 @@
+{{ define "user" }}
+<div class="padded box">
+    <div class="box-title">
+        <i class="mdi mdi-account-key"></i> changing {{ .SelectedUser.Name }}'s username
+    </div>
+    <form class="block" action="{{ printf "/admin/change_username_do?user=%s" .SelectedUser.Name | path }}" method="post">
+        <input type="text" id="username" name="username" placeholder="new username">
+        <input type="submit" value="change">
+    </form>
+</div>
+{{ end }}