clean up CI config

[vincerubinetti]

Our current CI config is a little bit "dirty" in that we have to set the permissions of a cache folder. I don't think it should have to be micromanaged like that. It should just be: install gh-pages and run it as a normal command.

Also, we're using circleci/node:latest in some places and an old version image of node in other places, and we should make that consistent.

This applies to hetmech-frontend, repurpose-frontend, disease-genes-frontend, and frontend-components.