Make number line draggable handle more discoverable

[catherinecarter]

During interviews, all three students struggled to find the interactive component on the number line (i.e. the scrubber). The following changes are proposed:

1. Remove the color behind each addend and the total (with color, they look interactive). Replace with a transparent white color in case the number is in front of other objects.
2. Have the color appear only when the handle is moved, like in the accordion box in Number Line: Integers
3. Make the scrubber handle larger
4. Make the pink color on the scrubber handle darker