Summary

Tests and minor fixes as follow up to #8138 , which was originally #8003, which closes #7957

• flexible layouts should use useFlexContainer
• modify useFlexContainer to use a ref to a size instead of an element, so that the timeline axis can be omitted from calculation
• do not allow for multiple time axes
• e2e tests for time strips
• fix existing e2e visual test where "Now Marker" isn't finished resizing before percy snapshot is taken. This is only a test artifact. App works as intended. See images.