Reduce clade count threshold

[trvrb]

This reduces the threshold for a clade to be included (and not wrapped into "other") from 5000 total sequences to 2000 total sequences. This causes clade 24B to be broken out in the current data.

There may be other changes to thresholds to pull in more geography, but I think this is a nice simple change to keep up with decreased sequencing volume.

[joverlee521]

Confirmed that 24B is now showing at https://nextstrain.org/sars-cov-2/forecasts