Show friendly message to unregistered users who view posts.

https://github.com/Growstuff/growstuff/issues/625

app/views/posts/_message.html.haml

@@ -0,0 +1,3 @@
+.row.alert.alert-success
+  .col-md-12
+    #{ post.author.login_name } is using Growstuff to discuss #{ post.subject } with a community of food gardeners worldwide. We have advice on growing hundreds of different #{link_to 'Crops', crops_url}, and a community from all around the world. #{link_to 'Sign In', new_member_session_url} or #{link_to 'Sign Up', new_member_registration_url} to respond to this post, or to start using Growstuff to track what you're planting and harvesting.

app/views/posts/show.html.haml

@@ -1,5 +1,8 @@
 = content_for :title, @post.subject
 
+- unless current_member
+  = render :partial => "message", :locals => { :post => @post }
+
 = render :partial => "single", :locals => { :post => @post, :subject => false, :hide_comments => true }
 
 - if can? :edit, @post or can? :destroy, @post