scrna UI improvements

[xzhou82]

• when the plot is launched in gdc it doesn't show sandbox. may take sample table out of nav toggle and show it above the plot
• when two plots are shown, show zoom icons per plot. when just one plot the icons are aligned with plot
• cell type legend click to show menu options: Hide/Show only/change color
• at navigation, conditionally hide Clusters tab if all plots colorColumns[] are all using the same term
• nav toggle tab name as Gene Expression, Differential Expression
• in GFF, on loading gene exp data, somehow shows two loading circles
• menu Sample size change to Dot Size
• increase default dot size to 0.02
• on gene exp overlay, by default use 95 percentile and let user customize by ColorScale
• label is "Open Violin" per Cluster (do not show drop down for Cluster since there's just one term; if there are multiple terms, show dropdown)
• when overlaying gene exp, should show <Gene> expression label on top of colorscale for user to understand

[airenzp]

0.02 makes the dots too big and covers other dots, see:

Used 0.008 which is better than before, maybe we could increase to 0.01