Make "Subscribe" button link to subscribe page for now

We're supposed to do fancy modals and all, but there's still time
for that later :P

src/components/Navbar.vue

@@ -80,7 +80,7 @@
           <Liberapay/>
         </a>
       </div>
-      <a v-if="Admin.site.subscribers" class="subscribe-button" href="#subscribe">Subscribe</a>
+      <a v-if="Admin.site.subscribers" class="subscribe-button" href="/subscribe">Subscribe</a>
       <a v-else class="rss-button" href="/feed.xml" title="RSS" target="_blank" rel="noopener">
         <RSS/>
       </a>