Add helper method for adding page specific styles for IDE-xterm

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

ee/app/views/ide/index.html.haml

-- content_for :page_specific_javascripts do
-  = stylesheet_link_tag "xterm.css"
+- add_page_specific_style "xterm.css"
 
 = render partial: "ide/show"