T3385
Many validation checks on Send screen -- together with desire to not show validation errors when user first loads the screen -- have made this set of conditions complex.
Store in single variable and add comments.
Disable send on page initial load state, which is not valid. Because `isNaN('') === false` (TIL), add condition for this check (prevents case of address being entered, value not yet entered -- button is enabled and you can still get a nonsense modal).