Create responsive grid modifiers

- create responsive grid modifiers (e.g. lap--grid--center)
- add 'pairs' for each of grid modifiers (for example,
`#{$class-type}#{$namespace}grid--spaced` as a pair to
`#{$class-type}#{$namespace}grid--full)
- create some modifier `pairs` only when namespace is not empty as the
CSS properties used would be the same as for `.grid` (for example,
generate `.lap--grid--spaced` but not `.grid--spaced`)
- rename `grid--rev` into `grid--rtr` so that there is a pair `#{$class-type}#{$namespace}grid--ltr` (i.e. `.lap--grid--ltr`)