Implement GEN.GetSeedTracks

[peddybeats]

• Re-use as much of the existing code as possible
• Call spotify.getTopTracks(accesstoken, timerange, numTracks) for each timerange
• 60% medium, 20% short, 20% long
• Select top 20 to send back down
• Equal weights in the first 10 and second 10

[peddybeats]

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