feat(teaching): cache course files with readable names

[umbopepato]

Closes #139

• Save course files with a combination of the original name, the id and the path inside the course folder
• Switch to DocumentDirectoryPath for local caching in order to make the cache visible to the user on iOS
• Add a migration to automatically move old cached files to the new structure
• Add a logic to automatically update cached files when the name/path changes remotely