Remove unusued site_settings, show checkbox in UI for boolean values, remove restrict_access

boolean to avoid locking yourself out by setting access_password to empty string. Minor
UI tweaks.

app/controllers/application_controller.rb

@@ -146,7 +146,7 @@ class ApplicationController < ActionController::Base
     return false if current_user.present?
 
     # Don't cache if there's restricted access
-    return false if SiteSetting.restrict_access?
+    return false if SiteSetting.access_password.present?
 
     true
   end
@@ -216,7 +216,7 @@ class ApplicationController < ActionController::Base
 
     def check_restricted_access
       # note current_user is defined in the CurrentUser mixin
-      if SiteSetting.restrict_access? && cookies[:_access] != SiteSetting.access_password
+      if SiteSetting.access_password.present? && cookies[:_access] != SiteSetting.access_password
         redirect_to request_access_path(:return_path => request.fullpath)
         return false
       end

app/controllers/invites_controller.rb

@@ -15,7 +15,7 @@ class InvitesController < ApplicationController
         user.enqueue_welcome_message('welcome_invite') if user.send_welcome_message
 
         # We skip the access password if we come in via an invite link
-        cookies.permanent['_access'] = SiteSetting.access_password if SiteSetting.restrict_access?
+        cookies.permanent['_access'] = SiteSetting.access_password if SiteSetting.access_password.present?
 
         topic = invite.topics.first
         if topic.present?

app/controllers/robots_txt_controller.rb

@@ -4,7 +4,7 @@ class RobotsTxtController < ApplicationController
   skip_before_filter :check_restricted_access
 
   def index
-    path = if SiteSetting.allow_index_in_robots_txt && !SiteSetting.restrict_access
+    path = if SiteSetting.allow_index_in_robots_txt && SiteSetting.access_password.blank?
       :index
     else
       :no_index