Adds the spacemacs|init-layer-company helper to set up company
completion for a language layer and replaces existing simple company
initializations. More complex initializations should use the new
architecture directly and remain untouched by this commit.
Move company and auto-complete to a common layer.
They are not enabled globally anymore, each mode using them
must explicitly declare a hook.
Only one frontend is supported for a given mode, we have to
choose the best between the two.
Only one key binding to toggle auto-completion on `SPC t a` no
matter if it is company or auto-complete. The lighter in the
mode-line is Ⓐ for both frontends.
