Fix cell menu getting stuck open.
Fix cell menu getting stuck open.

Fixes #1594

This also fixed an issue with a (N^2 + 1) binding leak on cell menu
click. This issue was due to an buggy implementation of a toggle
behavior and re-binding to existing DOM nodes over and over again without first unbinding.

The leaky bug was introduced in 92d2f9a7
PR: https://github.com/twosigma/beaker-notebook/pull/1073
ISSUE: To fix https://github.com/twosigma/beaker-notebook/issues/1070

brosander/beaker-notebook,gef756/beaker-notebook,mattyb149/beaker-notebook,ScottPJones/beaker-notebook,bradparks/beaker-notebook,sirinath/beaker-notebook,bradparks/beaker-notebook,sirinath/beaker-notebook,mattyb149/beaker-notebook,codeaudit/beaker-notebook,codeaudit/beaker-notebook,maxyeg/beaker-notebook,gef756/beaker-notebook,bradparks/beaker-notebook,ScottPJones/beaker-notebook,vital-ai/beaker-notebook,gef756/beaker-notebook,brosander/beaker-notebook,bradparks/beaker-notebook,mattyb149/beaker-notebook,sirinath/beaker-notebook,bradparks/beaker-notebook,vital-ai/beaker-notebook,Chasego/beaker-notebook,vital-ai/beaker-notebook,brosander/beaker-notebook,Chasego/beaker-notebook,vital-ai/beaker-notebook,gef756/beaker-notebook,ScottPJones/beaker-notebook,vital-ai/beaker-notebook,bradparks/beaker-notebook,sirinath/beaker-notebook,brosander/beaker-notebook,brosander/beaker-notebook,gef756/beaker-notebook,ScottPJones/beaker-notebook,Chasego/beaker-notebook,maxyeg/beaker-notebook,gef756/beaker-notebook,maxyeg/beaker-notebook,brosander/beaker-notebook,maxyeg/beaker-notebook,maxyeg/beaker-notebook,codeaudit/beaker-notebook,codeaudit/beaker-notebook,codeaudit/beaker-notebook,brosander/beaker-notebook,mattyb149/beaker-notebook,Chasego/beaker-notebook,bradparks/beaker-notebook,mattyb149/beaker-notebook,sirinath/beaker-notebook,Chasego/beaker-notebook,Chasego/beaker-notebook,vital-ai/beaker-notebook,ScottPJones/beaker-notebook,sirinath/beaker-notebook,codeaudit/beaker-notebook,mattyb149/beaker-notebook,vital-ai/beaker-notebook,sirinath/beaker-notebook,Chasego/beaker-notebook,mattyb149/beaker-notebook,gef756/beaker-notebook,maxyeg/beaker-notebook