Remove flash[:timedout] from build_flash_messages

[bikramwp]

In ActiveAdmin::Views::Pages::Base#build_flash_messages, skip the rendering for flash[:timedout]:

flash.each do |type, message|
  div message, :class => "flash flash_#{type}"
end

This is because devise uses the flash[:timedout] for redirection upon session time out and sets its value to true. If the flash is not removed, the resulting markup looks like this after a session time out:

<div class="flash flash_alert">Your session expired, please sign in again to continue.</div>
<div class="flash flash_timedout">true</div>

[jpmckinney]

Moved to #1048.