Remove prepare stage
Remove prepare stage

The prepare stage wasn't really doing too much for us since yarn is
going to cache most things anyway. Having it as a separate job just
added a few minutes before any build could run.

dartajax/frontend,jrjohnson/frontend,djvoa12/frontend,djvoa12/frontend,thecoolestguy/frontend,ilios/frontend,ilios/frontend,jrjohnson/frontend,thecoolestguy/frontend,dartajax/frontend