Add fit button to network visualization

[kemsguy7]

Closes #19

Note, originally this PR also made modifications from #23 . I've rewritten history to only include the network fit button feature, and added some related features/fixes of my (VR) own:

• VR: update table fixture data such that including the 4th "long" column in the table on the testbed page definitely overflows the screen width, testing scroll and collapse/expand CSS
• VR: make custom icons for expanding/collapsing table width and network viz width, since none in react-icons were quite adequate. some were close but felt not distinct enough from the new fit button icon added in this PR.
• KG: add network visualization fit button with icon
• VR: fix table expand/collapse css

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