Allow graders to select multiple sections at a time

[paulamiranda]

One usability improvement we'd like to request be added to SpeedGrader Upgrades is the ability for graders (any role who has the permission to grade) to be able to select multiple sections at once when using the Section filter. 

Currently, graders can only select one section at a time and must re-open the drop-down list to select additional sections (see gif below). The drop-down should remain open to allow graders to select the sections they need.

Having to click less will be especially helpful for graders in large enrollment courses that have many sections.

Drop-down list closes every time a section filter is selected/de-selected