Tweak signin / signup pages.

* Link to the signup page on the signin page.
* Clarify that emails are optional on the signup page.

app/views/sessions/new.html.erb

@@ -18,6 +18,10 @@
           <label for="remember" id="remember-label">Remember</label>
         </div>
 
+        <p class="hint">
+          New to <%= Danbooru.config.app_name %>? <%= link_to "Create a new account", new_user_path %>.
+        </p>
+
         <div class="input">
           <%= submit_tag "Submit", :data => { :disable_with => "Signing in..." } %>
         </div>