Do not store large arrays in session storage

[BearBearCodes]

Related to issue #2, the GUI should not be storing large arrays (i.e., passband response curves, spectra, 2D heatmaps) in the browser's session storage.

Instead, create state variables for each of these data. When the server returns the requested data, store the data in these variables instead. To ensure they persist after a page reload (and more generally, between sessions and browsers), we can check to see if the server has stored these data already and load them into their respective variables upon the initial page load (also see issue #6).