seo: mark login links as nofollow.

Mark links to the login page as rel="nofollow" so that search crawlers
don't constantly try to crawl it. Otherwise the fact the login url is
different on every page (/login?url=<current_url>) confuses crawlers.

Also strip the url param from the canonical url (<link rel="canonical">)
on the login page.

app/views/layouts/_main_links.html.erb

@@ -1,6 +1,6 @@
 <menu id="main-menu" class="main">
   <% if CurrentUser.is_anonymous? %>
-    <%= nav_link_to("Login", login_path(url: request.fullpath)) %>
+    <%= nav_link_to("Login", login_path(url: request.fullpath), rel: "nofollow") %>
   <% else %>
     <%= nav_link_to("My Account #{unread_dmail_indicator(CurrentUser.user)}", profile_path) %>
   <% end %>

app/views/sessions/new.html.erb

@@ -1,5 +1,6 @@
 <% page_title "Login" %>
 <% meta_description "Login to #{Danbooru.config.app_name}" %>
+<% canonical_url login_url %>
 
 <%= render "sessions/secondary_links" %>