Activate lfs for sketch files

Following the question on slack https://gitlab.slack.com/archives/C03MSG8B7/p1538508503000100

@gitlab-com/gitlab-ux does anyone know why this wasn't done up until now?

@ahanselka I remember you helping us out when we had some problems with lfs in the past (might it be worthwhile to convert all existing sketch files to lfs stored ones?)