Alphabetize skills on learn page

See https://github.com/codeforamerica/bizfriendly-web/issues/110

Categories (skills) are alphabetized with a sort function. [Details at MDN.](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort?redirectlocale=en-US&redirectslug=JavaScript%2FReference%2FGlobal_Objects%2FArray%2Fsort)

Also, determining what column a skill should be listed in based on its index in the array was unreliable, because there are unpublished skills in it and that created holes in the column. I fixed this by creating an array of only published skills first.