Rework application configuration

Use a preinitializer to load the settings from application.yml so
that they are available as early as possible. All settings can also
be overridden using environment variables.

The ad-hoc settins in environment.rb are then moved to this new
system so we have one consistent location for settings.

app/views/trace/_trace.html.erb

@@ -1,7 +1,7 @@
 <tr>
   <% cl = cycle('table0', 'table1') %>
   <td class="<%= cl %>">
-    <% if OSM_STATUS != :gpx_offline %>
+    <% if STATUS != :gpx_offline %>
       <% if trace.inserted %>
         <a href="<%= url_for :controller => 'trace', :action => 'view', :id => trace.id, :display_name => trace.user.display_name %>"><img src="<%= url_for :controller => 'trace', :action => 'icon', :id => trace.id, :display_name => trace.user.display_name %>" border="0" alt="" /></a>
       <% else %>

app/views/trace/view.html.erb

@@ -1,6 +1,6 @@
 <h2><%= t 'trace.view.heading', :name => h(@trace.name) %></h2>
 
-<% if OSM_STATUS != :gpx_offline %>
+<% if STATUS != :gpx_offline %>
   <% if @trace.inserted %>
     <img src="<%= url_for :controller => 'trace', :action => 'picture', :id => @trace.id, :display_name => @trace.user.display_name %>">
   <% else %>