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

Dockerfile

@@ -11,7 +11,7 @@ ARG TAGS="sqlite sqlite_unlock_notify"
 ENV TAGS "bindata $TAGS"
 
 #Build deps
-RUN apk --no-cache add build-base git
+RUN apk --no-cache add build-base git nodejs npm
 
 #Setup repo
 COPY . ${GOPATH}/src/code.gitea.io/gitea