Refine object constancy, handling of unfound genes, subpart hover text

[eweitz]

This lets users more easily visually track exons when splicing. It also fixes an edge-case bug in the related genes kit.

Previously, the exon splice animation completed in 250 ms. That's a bit too fast to track which transcript subparts end up where after splicing exons in or out. Now, the animation is slowed down 3x, to 750 ms, making it realistic to follow moving parts with your eyes.

Here's how it looks -- old low constancy in first scene, new high constancy in second scene:

https://user-images.githubusercontent.com/1334561/201238942-5ae1ccad-35ca-46b0-b05f-5da4734a4299.mp4

Also, the "Related genes" example now has better handling of unfound genes. Before, search a known gene (e.g. "ACE2") after an unknown gene ("asdf") threw a blocking error. It required refreshing the page to fix; not good. Now, searching known after unknown seamlessly returns expected results for the known searched gene.

Finally, subpart hover text was also trimmed for slightly less visual noise.

[coveralls]

Coverage remained the same at 86.219% when pulling 532a420077234d561435fe51aa29c87262fbe843 on object-constancy-etc into 07744ddf206860e4af5b091f33d2ac319da4b44d on master.