Add fit button to network visualization (#30)

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

Co-authored-by: Vincent Rubinetti <[email protected]>

Author: kemsguy7