Consider storage limits and what happens when those are hit

[rozbb]

Currently, RTMS assumes it has infinite space for audio files. It would be wise to consider a way to limit space usage (either globally or per-user), and consider what happens when it's hit.

For example, a user who hits their space limit might not be able to add any articles until they "archive" some. Archiving might amount to deleting the MP3 file but saving the metadata somewhere. The the library, there might be an extra section for archived articles. It shows their title and source, but no Add to Queue button.