Use standard lessc and minify CSS using Node.js (#2337)

* Use standard lessc and minify CSS using Node.js

This changes the previous nonstandard `lessc` to the official one and
enables CSS minification via the clean-css module.

To build CSS, Node.js is required along with a `npm install` to get the
tools installed locally in node_modules so there is no dependency on
binaries in PATH. Benefits include:

- Allows one to have a standard lessc in PATH.
- Can now use command line switches on lessc.
- Minified CSS brings faster page load times and also has the benefit
  of discouraging contributors from editing CSS directly.

To build CSS, Node.js is required along with a `npm install` to get the
tools installed locally based on the information in `package.json`.

The 'make stylesheet' task was modified to run without condition. This
makes it easier to work on the make task itself without having to delete
files.

Also fixes: https://github.com/go-gitea/gitea/issues/2198

* install node, npm and modules on drone

* .PHONY

* use 'minify' to minify CSS

CONTRIBUTING.md

@@ -97,6 +97,16 @@ and is synced regularily to Crowdin. Once a translation has reached
 A SATISFACTORY PERCENTAGE it will be synced back into this repo and
 included in the next released version.
 
+## Building Gitea
+
+Generally, the go build tools are installed as-needed in the `Makefile`.
+An exception are the tools to build the CSS and images.
+
+- To build CSS: Install [Node.js](https://nodejs.org/en/download/package-manager)
+  with `npm` and then run `npm install` and `make stylesheets`.
+- To build Images: ImageMagick, inkscape and zopflipng binaries must be
+  available in your `PATH` to run `make generate-images`.
+
 ## Code review
 
 Changes to Gitea must be reviewed before they are accepted, no matter who