Add Node.js build dep, remove built js/css files (#9114)

- Added Node.js as build dependency and removes build files from git.
- Added version checks for both Go and Node.js.
- Overhauled the js/css make target to only run when needed.
- Merged the `generate` make target into `build` as per suggestion.

Fixes: https://github.com/go-gitea/gitea/issues/6782
Fixes: https://github.com/go-gitea/gitea/issues/9216

package.json

@@ -2,7 +2,7 @@
   "license": "MIT",
   "private": true,
   "engines": {
-    "node": ">=8"
+    "node": ">=10"
   },
   "devDependencies": {
     "@babel/core": "7.7.2",