controls/color-by: Use placeholders instead of a default gene/position
controls/color-by: Use placeholders instead of a default gene/position

Default gene (HA1 or nucleotide) and position (1) selections are
replaced by informative placeholders and nothing changes until a full
selection is made.

If an invalid position is entered, the app remains at the last good
state instead of resetting back to a default state.  This allows a
correction of the position entered instead of requiring a re-entering.

The previous placeholders rarely or never triggered because default
values were set.

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice