Animate opened folder on load in the Web IDE

When the Web IDE is launched directly to a folder we should provide a visual cue so that the user knows where the context is.

Further details

(Identified in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19887#note_83134509)

might it be possible to let the folder that was loaded, have a short "active" state animation. Meaning that the background of the list item initially has the background that fades away (as a folder can't really be active).

This might make the user more aware that we opened that folder :). Please use an out fade curve if possible :)