Fix slider initial render bug
Fix slider initial render bug

This was caused by the _handleResize method updating internal component state causing a rerender. This rerender wasn't being picked up by React reconciliation properly (I couldn't figure out why). The slider never needs to adjust responsively, and having this responsive adjustment was triggering unnecessary component rerenders during app use. Here, I removed _handleResize and manually set slider width parameters.
nextstrain/auspice,nextstrain/auspice,nextstrain/auspice