Set default progress value to 0

Set the value of the progress element for “Download grades” to 0 (was
22) so that there isn’t a flash of the progress bar at 22 before it
gets set to the actual value.
Display progress bar for grades download

Added code to set the “Download grades” progress bar’s max and value
attributes, reflecting the progress being made in downloading grades
data for enrolled users in a course.

Also expanded the CSS class `d2l-wait-download` into
`d2l-wait-download-classlist` and `d2l-wait-download-gradesdata` to
differentiate between these two steps. The existing indeterminate
spinner is shown for `d2l-wait-download-classlist` while the progress
bar is shown for `d2l-wait-download-gradesdata`.