Add frontend/backend make targets, fix source release (#10325) (#10414)

* Add frontend/backend make targets, fix source release

- Add 'make backend' and 'make frontend' make targets which are used to
  build go and js/css/svg files respectively.

- The 'backend' target can be invoked without requiring Node.js to be
  present on the system if pre-built frontend assets are present like
  in the release source tarballs.

- Fix source releases missing 'dist' folders inside 'node_modules' which
  were erronously excluded from tar.

- Store VERSION in file VERSION for the release tarballs and prefer that
  file over git-derived version.

* fix release task

* fix typo

* fix another typo

Fixes: https://github.com/go-gitea/gitea/issues/10253

README.md

@@ -33,6 +33,15 @@ From the root of the source tree, run:
 
     TAGS="bindata" make build
 
+The `build` target is split into two sub-targets:
+
+- `make backend` which requires [Go 1.11](https://golang.org/dl/) or greater.
+- `make frontend` which requires [Node.js 10.0.0](https://nodejs.org/en/download/) or greater.
+
+If pre-built frontend files are present it is possible to only build the backend:
+
+    TAGS="bindata" make backend
+
 More info: https://docs.gitea.io/en-us/install-from-source/
 
 ## Using