Make rich text edit controls use as much space as possible

Use CSS3 calculations to set the width of a rich text edit control
to the maximum possible while still allowing space for the help.

Also set the subject/title fields to the same width.

Fixes #657

app/views/message/new.html.erb

@@ -8,7 +8,7 @@
   <fieldset>
     <div class='form-row'>
       <label class="standard-label"><%= t'message.new.subject' %></label>
-      <%= f.text_field :title, :size => 60, :value => @subject %>
+      <%= f.text_field :title, :size => 60, :value => @subject, :class => "richtext_title" %>
     </div>
     <div class='form-row'>
       <label class="standard-label"><%= t'message.new.body' %></label>