Make save/cancel button in milestone selector within issue boards sticky in dropdown

Resources

FE @iamphill

Problem

Currently the save and cancel button in the dropdown for selecting milestones which will be defaulted to a specific issue board are scrolling with the content.

Proposal

They should be sticky and always available

cc: @lbennett @victorwu

similar to the sticky bottom menu option in for example this menu:

Good idea! That dropdown did throw me off when I first used it. We should probs try get this into %9.3?

@jschatz1 : Small thing if you want to throw it into 9.3.

Yess, threw me off as well. Great improvement!

added Deliverable label

changed milestone to %9.4

changed the description

assigned to @iamphill

mentioned in merge request !14785