improve no-member NAT warning

public/css/main.css

@@ -108,7 +108,7 @@ div.success {
 div.warn {
     color: #856404;
     background: #fff3cd;
-    border-color: #ffeeba;
+    border-color: #f2d376;
 }
 
 div.error {

public/index.html

@@ -24,7 +24,7 @@
             <div id="error-proxy" class="error">You are not a StuStaNet member and did not configure the proxy server. <a href="http://conntest.stustanet.de/nomember">Click here for further information</a>.</div>
             <div id="error-blocked" class="error">Your Internet Access is blocked due to malware or malicious behavior. <a href="http://conntest.stustanet.de/blocked">Click here to view the blocking page with more information</a>.</div>
             <div id="error-unknown" class="error">An unknown error occurred. Please contact a network administrator and attach the log at the bottom of this page.</div>
-            <div id="warn-nat" class="warn">Network Address Translation (NAT) is not working, meaning that many application will not work as expected. However, this is expected, as you do not seem to be a StuStaNet member. NAT is a members-only service.</div>
+            <div id="warn-nat" class="warn">Network Address Translation (NAT) is not working, meaning that many application will not work as expected. However, this is expected, as you do not seem to be a StuStaNet member. NAT is a members-only service.<br />You can <a href="https://reg.stustanet.de/">become a member here.</a></div>
             <div id="status">Initializing...</div>
             <table id="tests">
                 <thead>