* Support npm@5 and yarn
Use package.default.json
Partial #6008
- Overwrite package.json with package.default.json values
- `dependencies` field is merged with package.default.json version taking precidence
- `./nodebb upgrade` automatically does those things and runs `git pull`
- use `./nodebb upgrade --dev` to avoid the `git pull`
* added logic to preserve extraneous plugins installed in node_modules/
* Don't automatically git pull
* Simplify package-install, run it on upgrade just in case
