Donation alert tiers

[Planks]

Add (to nodecg webadmin) tiers of audio/visual alerts instead of having a fixed alert ("the ding").

Pseudocode:

alerts [
threshold: 0, sound: alert.mp3, volume: 50, graphic: none
threshold: 5, sound: 5dollar.mp3, volume: 50,  graphic: none
threshold: 15, sound: 15dollar.mp3, volume: 80, graphic: meme.png
threshold: 50, sound: 50dollar.mp3, volume: 100,  graphic: omegameme.gif
]

threshold: The minimum currency amount passed from the tracker for the alert to play. 0 is valid (any donation).
sound: the sound that should play (taken from uploads)
volume: modifier as a %, allows us to balance the sfx if they're not normalised and also allows us to make them progressively louder. values above 100 allowed (to add gain, suggest capping at 250)
graphic: an additional graphic that should immediately replace the mediabox (taken from uploads)

Either sound, graphic, or both, are required.

[zoton2]

Planks (in Discord DMs) has also suggested another parameter: graphic display time.

[Planks]

Now partially completed with audio effects as defined