Cannot select all cells after re-embedding and labeling a subset in a new category

[ktravaglini]

Related to #280

cellxgene version:
(cellxgene) DNab4216c1:scanpy ktrav$ cellxgene --version
[cellxgene] Version 0.15.0

Launch command used:
cellxgene launch --experimental-enable-reembedding droplet-infection_1-adata_stroma-cxg.2020-06-15.h5ad

Steps to reproduce:

1. Select a group of cells, subset, and re-embed
2. Change to new embedding
3. Select another group of cells
4. Create a new category
5. Add label to the category and select middle "Add X number of selected cells to this label"

After following this sequence, I am unable to select any cells not in the group I just labeled. I do not believe this behavior is intended because this does not occur if you select and label cells using the original embedding.

Undoing the subset (clicking the whole circle icon) allows selection again, but is annoying if labeling multiple groups on the new embedding.