Re-add track when track exists in removed state

[Noam-Bendelac]

In the POST .../:playlistId/tracks/ endpoint, need to check if the track already exists in the db (in the removed state), and if so update the db differently.

[Noam-Bendelac]

Or alternatively, the POST .../:playlistId/tracks/ endpoint shouldn't be used for re-adding after removal; rather the PUT .../:trackId/removed endpoint should be used. The frontend would need to enforce & follow this. (Maybe have the POST .../:playlistId/tracks/ endpoint check if the track exists in the db and if so, respond with an error).

[Noam-Bendelac]

For milestone 1, we don't need to worry about this because track removal will make it as if the track was never there. Milestone 2 will need to worry about this.