New key bindings, new function to disable popwin for helm buffers

• C-j and C-k can be used to select auto-complete candidates (evil-insert-digraph in insert mode has been moved to C-i)
• <SPC> w p m shows the *Messages* buffer in a popup window, <SPC> w p p close the current popup window.
• New function spacemacs/remove-popup-display-config which can be used to disable popup windows for some modes (i.e. helm). More info in the tip of the readme file: https://github.com/syl20bnr/spacemacs#tips-for-spacemacs-advanced-users