I've been working on sorting all the assets which were floating around the directory into dedicated lesson folders, then updating the links within the lessons to reflect the change.

Some notes:

• I've left the python-lessons and python-es-lecciones . zip folders as they were (floating), because I believe it may make more sense that way. Please let me know if you disagree!
• I deleted two files:
  • galdos_miau.txt: a copy of this file was already sorted into en/sentiment-analysis-syuzhet. However, because the original lesson here is the ES version es/analisis-de-sentimientos-r, I moved it to a new folder under that slug. I then deleted the extra copy which was just floating.
  • scissorsandpaste-master.zip: no file under this name was use in any of our lessons. However, it is identical to master.zip, which was already sorted in the en/transforming-xml-with-xsl folder.

Closes #3441

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