Add legacy mode that pre-maps Extended Attributes

[tvillega]

There are course blocks on the jpeg's like Termodinámica / Termodinámica Química handled by a special code on Malla API that maps back to Extended Attributes.

To maximize compatibility with existent projects of malla frontend, add a legacy mode that pre-maps the course and returns a block the same way as the original jpeg shows it.

This comes could work by appending the query string xattr=0 on each request and could be handled by the server.