Upgrade prettier to latest (v1.18.2)

Upgrades prettier to the latest version, and auto-corrects files with yarn prettier-all-save

The diffs all seem to stem from formatting changes in brackets within template literals. Nothing major.

Relevant changelogs here:
https://prettier.io/blog/2019/04/12/1.17.0.html
https://prettier.io/blog/2019/06/06/1.18.0.html

CE MR: gitlab-org/gitlab-ce!29943