mgr/dashboard: Improve prettier scripts and documentation

mgr/dashboard: Improve prettier scripts and documentation

Added prettier validation to the run-frontend-unittests script. This will make
sure we are always running prettier in our commits.

Added 2 new npm scripts:
- 'prettier', will run prettier formatter on all frontend files
- 'prettier:lint', will check all frontend files against prettier linter

Removed 'pretty-quick' and related scripts. Since we now have all files
prettified we can simply run prettier on them.

Remove 'tslint-eslint-rules' package and all related rules. Prettier can check
all the removed rules.

Updated HACKING.rst with some information about prettier.

Signed-off-by: Tiago Melo <tmelo@suse.com>