Mark buttons active if corresponding form is shown in epics tree

The following discussion from !17932 (merged) should be addressed:

• @annabeldunstone started a discussion: (+3 comments)

  When an item is selected, can we add the active state to it as discussed here #5419 (comment 225183533)

  At the moment it's not as clear which option you've selected (this is already true of create vs add new epic)