CSS: add new application layout + CSS

Without bootstrap!

app/assets/stylesheets/application.scss

@@ -33,13 +33,11 @@
 // = require etapes
 // = require fonts
 // = require france_connect_particulier
-// = require header
 // = require landing
 // = require left_panel
 // = require login
 // = require main_container
 // = require navbar
-// = require new_footer
 // = require notification_alert
 // = require pieces_justificatives_fields
 // = require pj_modal