🔒️ Escape requested user id

Fixes an XSS issue.

src/octoprint/templates/login.jinja2

@@ -69,12 +69,12 @@
             <div id="login-offline" class="alert alert-error">{{ _('Server is currently offline.') }} <a id="login-reconnect" href="javascript:void(0)">{{ _('Reconnect...') }}</a></div>
 
             {% if user_id %}<p>
-                {{ _('The following account is required:') }} {{ user_id }}
+                {{ _('The following account is required:') }} {{ user_id|e }}
             </p>{% elif logged_in %}<p>
                 {{ _('An account with the following permissions is required:') }} {{ permission_names|join(", ") }}
             </p>{% endif %}
 
-            <input type="text" id="login-user" data-test-id="login-username" class="input-block-level" placeholder="{{ _('Username')|edq }}" {% if user_id %}value="{{ user_id }}" disabled{% endif %} autofocus autocapitalize="none">
+            <input type="text" id="login-user" data-test-id="login-username" class="input-block-level" placeholder="{{ _('Username')|edq }}" {% if user_id %}value="{{ user_id|edq }}" disabled{% endif %} autofocus autocapitalize="none">
             <input type="password" id="login-password" data-test-id="login-password" class="input-block-level" placeholder="{{ _('Password')|edq }}">
             <span class="pull-right"><small><a href="https://faq.octoprint.org/forgotten-password" id="login-forgotpassword" target="_blank" tabindex="-1">{{ _('Forgot password?') }}</a></small></span>
             <label class="checkbox">