Reduce the number of terminals required to build riot-web to 1

A step towards a real solution for https://github.com/vector-im/riot-web/issues/7305

This approach makes use of `npm link` to remove the use of symlinks in the build process. The build process has also been altered to invoke the build process of each underlying SDK (react, js). This means that one can now `npm link` and `npm start` and have a working environment. 

At the same time, parallelshell was dropped due to lack of maintenance from the maintainer.

scripts/fetch-develop.deps.sh

@@ -52,13 +52,7 @@ function dodep() {
     echo "$repo set to branch "`git -C "$repo" rev-parse --abbrev-ref HEAD`
 
     mkdir -p node_modules
-    rm -r "node_modules/$repo" 2>/dev/null || true
-    ln -sv "../$repo" node_modules/
-
-    (
-        cd $repo
-        npm install
-    )
+    npm link "../$repo"  # This does an npm install for us
 }
 
 ##############################
@@ -77,14 +71,15 @@ echo 'Setting up matrix-react-sdk'
 
 dodep matrix-org matrix-react-sdk
 
-# replace the version of js-sdk that got pulled into react-sdk with a symlink
+# replace the version of js-sdk that got pulled into react-sdk with a link
 # to our version. Make sure to do this *after* doing 'npm i' in react-sdk,
 # otherwise npm helpfully moves another-json from matrix-js-sdk/node_modules
 # into matrix-react-sdk/node_modules.
 #
 # (note this matches the instructions in the README.)
-rm -r node_modules/matrix-react-sdk/node_modules/matrix-js-sdk
-ln -s ../../matrix-js-sdk node_modules/matrix-react-sdk/node_modules/
+cd matrix-react-sdk
+npm link ../matrix-js-sdk
+cd ../
 
 echo -en 'travis_fold:end:matrix-react-sdk\r'