add 'npm' and 'npm-update' make targets and lockfile (#7246)

* add 'npm' and 'npm-update' make targets and lockfile

- `make npm` installs and updates node_modules, triggered automatically
   on `make css` and `make js` as it completes reasonably fast and
   ensures consistent modules.
- `make npm-update` updates all dependencies to their latest version,
   regenerates `node_modules` from scratch and updates
   `package-lock.json`. It uses npm modules `updates` written by yours
   truly to find the latest version of each dependency.

* add suggested make dependencies

* remove package-lock.json during npm-update

* regenerate package-lock.json

doc/advanced/hacking-on-gitea.en-us.md

@@ -138,14 +138,7 @@ make vet lint misspell-check
 
 ### Updating CSS
 
-To generate the CSS, you will need [Node.js](https://nodejs.org/) 8.0 or greater and the build dependencies:
+To generate the CSS, you will need [Node.js](https://nodejs.org/) 8.0 or greater with npm. At present we use [less](http://lesscss.org/) and [postcss](https://postcss.org) to generate our CSS. Do **not** edit the files in `public/css` directly, as they are generated from `lessc` from the files in `public/less`.
-
-```bash
-npm install
-```
-
-At present we use [less](http://lesscss.org/) and [postcss](https://postcss.org) to generate our CSS. Do
-**not** edit the files in `public/css` directly, as they are generated from `lessc` from the files in `public/less`.
 
 Edit files in `public/less`, run the linter, regenerate the CSS and commit all changed files:
 
@@ -155,13 +148,7 @@ make css
 
 ### Updating JS
 
-To run the JavaScript linter you will need [Node.js](https://nodejs.org/) 8.0 or greater and the build dependencies:
+To run the JavaScript linter you will need [Node.js](https://nodejs.org/) 8.0 or greater with npm. Edit files in `public/js` and run the linter:
-
-```bash
-npm install
-```
-
-Edit files in `public/js` and run the linter:
 
 ```bash
 make js