This change updates a few more dependencies, and gets rid of some things

that we actually don't need:

Passport has been updated to the latest, and the slack stragegy has

  been switched over to a different implementation which is actually

  being maintained.
Changed to importing the standalone build for selectize, which lets us

  get rid of the explicit sifter dependency. sifter was depending on

  some vulnerable versions of other libraries. The standalone Selectize

  bundles a newer version of selectize which doesn't have this problem.
The published versions of scss libraries have some security problems.

  We're not importing any SCSS files, so all of this can just go away.
We're no longer using gulp-shell at all.

Verified that login/logout still worked correctly. Checked build and

basic functionality.