Start to isolate the list of packages. Also we want to define packages
function at the top level of an elisp file in order to not prevent
emacs from knowing where the functions are defined.
This commit has a POC made with elfeed and elixir layers.
Rename variable `elfeed-web-enabled-on-emacs-startup` to
`elfeed-enable-web-interface`
Change screenshot for a logo
Fix loading of elfeed when elfeed-enable-web-interface is non nil
Replace deprecated functions calls like `evil-leader/set-key` or
`spacemacs|evilify-map`.
