Fix session[:edit] breakage in picture controller.

When picture controller is accessed the current content of
session[:edit] is lost. This is due to asymetry in
get_global_session_data vs set_global_session_data. One side always sets
the session[:edit] while the other does not always read it.

Furthemor the logic of {set/get}_global_session_data is not needed in
the picture controller.

app/controllers/picture_controller.rb

@@ -1,4 +1,7 @@
 class PictureController < ApplicationController
+  skip_before_action :get_global_session_data
+  skip_after_action :set_global_session_data
+
   def show # GET /pictures/:basename
     compressed_id, extension = params[:basename].split('.')
     picture = Picture.find_by_id(from_cid(compressed_id))