Replace bower with yarn

[jmontoyaa]

bower will be deprecated

See:
bower/bower#2298

better to use

https://yarnpkg.com/en/

Change could be done for 2.0

[jmontoyaa]

I added a readme file here:

https://github.com/chamilo/chamilo-lms/blob/master/assets/README.md

[jmontoyaa]

Related to #2324