Add top gradient to boards selector when scrolled down

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3384 we proposed adding a gradient to the bottom of the boards selector dropdown list to show that the container is scrollable (specifically, can be scrolled down).

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3384#note_48634978 @pedroms suggested we include a gradient that the top of the container to show that it can be scrolled up.

/cc @pedroms @cperessini @victorwu