Add field to hide tracks from charts

[echarlie]

From @mutantmonkey on November 10, 2016 7:45

Some talk shows have ended up in the charts because they log the same thing every week, even though they're not playing music. While it's useful to have these logged, keeping them off the charts is ideal.

This field should only be editable from the library interface and not exposed to DJs directly.

Copied from original issue: wuvt/wuvt-site#224

[echarlie]

We have a lot of tracks with semi-nonsense names now: https://www.wuvt.vt.edu/playlists/track/323725 and https://www.wuvt.vt.edu/playlists/track/502631 are good examples. These spins are almost exclusively by automation-as-DJ (i.e. pload), mostly have "sensible" album names, and can be included in the tracklog and optionally the last15 (although see #13). So it might be better to hide artists or hide albums, instead of tracks.