Use variables for list-group-item color levels

[yankee42]

The color levels used in alerts:

bootstrap/scss/_alert.scss

Line 49 in c13ec3e

@include alert-variant(color-level($value, $alert-bg-level), color-level($value, $alert-border-level), color-level($value, $alert-color-level));

And in tables:

bootstrap/scss/_tables.scss

Line 105 in c13ec3e

@include table-row-variant($color, color-level($value, $table-bg-level), color-level($value, $table-border-level));

Come from a variable. However the color level for list-group-item is hard coded:

bootstrap/scss/_list-group.scss

Line 159 in c13ec3e

@include list-group-item-variant($color, color-level($value, -9), color-level($value, 6));

This does strike me as inconsistent. I think all these options should be equally customizable. This PR fixes this by also putting the color levels for list-group-items into a variable.

Also note, that the default value for alert&table are both -10 and 6 for background and color respectively. However the defaults for list-group-item ist -9 and 6 for background and color respectively. I did not change this (yet) as I am not sure whether this might be on purpose.