honor the 'viewport' of direct links (dont autofit them away)

[melange396]

• closes Preserve saved "viewport" graph extents in URL-encoded links #96

Epivis direct links (with dataset information encoded in an epivis url) can include a "viewport" parameter that represents the graph axis extents at time of link generation. The recently added "autofit" mode functionality broke this, by automatically zooming to see the entirety of all the current signals on load. This PR should now restore the saved viewport when it is present, or default to autofit behavior when viewport is not provided in the URL.

[melange396]

zoomed view with saved viewport:
https://deploy-preview-114--cmu-delphi-epivis.netlify.app/#eyJjaGFydCI6eyJ2aWV3cG9ydCI6WzczNzMyOC43NTkxOTIzNjk1LDAuNzI0NDEzNzYxOTQxOTc5Nyw3MzgzMzkuNDI5OTA2ODE1NCwxLjcyODk3ODk3MzUwNzUzMV0sInNob3dQb2ludHMiOmZhbHNlfSwiZGF0YXNldHMiOlt7ImNvbG9yIjoiIzhiYjNiMSIsInRpdGxlIjoiJXdJTEkiLCJwYXJhbXMiOnsiX2VuZHBvaW50IjoiZmx1dmlldyIsInJlZ2lvbnMiOiJuYXQifX1dfQ==
(url encodes:

{"chart":{"viewport":[737328.7591923695,0.7244137619419797,738339.4299068154,1.728978973507531],"showPoints":false},"datasets":[{"color":"#8bb3b1","title":"%wILI","params":{"_endpoint":"fluview","regions":"nat"}}]}

)

same but with viewport NULL:
https://deploy-preview-114--cmu-delphi-epivis.netlify.app/#eyJjaGFydCI6eyJ2aWV3cG9ydCI6bnVsbCwic2hvd1BvaW50cyI6ZmFsc2V9LCJkYXRhc2V0cyI6W3siY29sb3IiOiIjOGJiM2IxIiwidGl0bGUiOiIld0lMSSIsInBhcmFtcyI6eyJfZW5kcG9pbnQiOiJmbHV2aWV3IiwicmVnaW9ucyI6Im5hdCJ9fV19
(url encodes:

{"chart":{"viewport":null,"showPoints":false},"datasets":[{"color":"#8bb3b1","title":"%wILI","params":{"_endpoint":"fluview","regions":"nat"}}]}

)

same but with no viewport:
https://deploy-preview-114--cmu-delphi-epivis.netlify.app/#eyJjaGFydCI6eyJzaG93UG9pbnRzIjpmYWxzZX0sImRhdGFzZXRzIjpbeyJjb2xvciI6IiM4YmIzYjEiLCJ0aXRsZSI6IiV3SUxJIiwicGFyYW1zIjp7Il9lbmRwb2ludCI6ImZsdXZpZXciLCJyZWdpb25zIjoibmF0In19XX0=
(url encodes:

{"chart":{"showPoints":false},"datasets":[{"color":"#8bb3b1","title":"%wILI","params":{"_endpoint":"fluview","regions":"nat"}}]}

)

live epivis site with saved viewport (same url hash and encoded payload as the top link ^ but this does the autofit and does not preserve the viewport):
https://delphi.cmu.edu/epivis/#eyJjaGFydCI6eyJ2aWV3cG9ydCI6WzczNzMyOC43NTkxOTIzNjk1LDAuNzI0NDEzNzYxOTQxOTc5Nyw3MzgzMzkuNDI5OTA2ODE1NCwxLjcyODk3ODk3MzUwNzUzMV0sInNob3dQb2ludHMiOmZhbHNlfSwiZGF0YXNldHMiOlt7ImNvbG9yIjoiIzhiYjNiMSIsInRpdGxlIjoiJXdJTEkiLCJwYXJhbXMiOnsiX2VuZHBvaW50IjoiZmx1dmlldyIsInJlZ2lvbnMiOiJuYXQifX1dfQ==