Combine todo, subscribe, move functions in the issue sidebar

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/34261#note_36666376.

added UX label

mentioned in issue #34261 (closed)

@dimitrieh : Shouldn't at least Mark done and Subscribe be buttons? And I think Move really depends on the final design of https://gitlab.com/gitlab-org/gitlab-ce/issues/34261.

How would this behave when the entire sidebar is collapsed? Right now the subscribe icon in collapsed mode is at the bottom. I assume when it's collapsed, we should move the collapsed button to the top, right under the todo button to be consistent.

That whole collapsed sidebar needs some rework. The icons are changing right?

Some of the copy is inconsistent and the there's a random horizontal rule near the bottom.

Just a note that we have plans to change the subscribe button to a toggle: https://gitlab.com/gitlab-org/gitlab-ce/issues/16988