Tweak setting initialization

I could not see a meaningful reason behind calling `forbid` so early in
the settings initialization block. But moving the `forbid` call until
later resolved this issue.

In the real world, I don't see how this would cause issues. But I found
that calling `forbid` when `wiki_options[:allow_editing]` was set to
false caused order-dependent test errors where Sprockets would end up
being badly configured and left without an initialized
`Sprockets::Environment` object, which is required by the
`sprockets-helpers` gem to resolve asset paths.

The test that would cause errors is also in this commit diff. I've
updated it to be a bit more readable.

I also took this opportunity to review and clean up the `@allow_editing`
assignment, which seemed a bit obfuscated.

lib/gollum/app.rb

@@ -103,8 +103,7 @@ module Precious
     end
 
     before do
-      settings.wiki_options[:allow_editing] = settings.wiki_options.fetch(:allow_editing, true)
-      @allow_editing = settings.wiki_options[:allow_editing]
+      @allow_editing = settings.wiki_options.fetch(:allow_editing, true)
       @critic_markup = settings.wiki_options[:critic_markup]
       @redirects_enabled = settings.wiki_options.fetch(:redirects_enabled, true)
       @per_page_uploads = settings.wiki_options[:per_page_uploads]
@@ -112,8 +111,6 @@ module Precious
       
       @wiki_title = settings.wiki_options.fetch(:title, 'Gollum Wiki')
 
-      forbid unless @allow_editing || request.request_method == 'GET'
-
       if settings.wiki_options[:template_dir]
         Precious::Views::Layout.extend Precious::Views::TemplateCascade
         Precious::Views::Layout.template_priority_path = settings.wiki_options[:template_dir]
@@ -143,6 +140,8 @@ module Precious
           config.manifest = Sprockets::Manifest.new(settings.sprockets, @static_assets_path)
         end
       end
+
+      forbid unless @allow_editing || request.request_method == 'GET'
     end
 
     get '/' do