Graph vis app: fix glitching labels

[eugeniashurko]

When the visualization app is used (after some point: switching between graphs, merging removing nodes) the node labels start to glitch on zoom in/out. They disappear and/or are not displayed correctly.

This bug needs to be further investigated in order to isolated the scenarios when the glitching appears. It is possible that the source is some internal dash-cytoscape bug.