Remove scala-mode2 and install via package.el

* Fully delete the previously added submodule scala-mode2
* Remove the lines from config/prog.el
* Install using package.el and update the requirements.txt file
Remove yasnippet and install via package.el

* Fully remove the yasnippet submodule
* Update config
* Update requirements.txt file
Remove nhtml submodule (not used anymore)
Install js2-mode and update config
Fix nd-js.el to work with js2-mode