Work item threads/comments - make add actions appear as inputs

Follow up from #389831 (closed)

This is intended to mirror notes on Issues, MRs, Epics, etc. Unbox comment UI (#381727) can be a later follow up across those same locations.

Changes to comments in WI notes

Make top level comments open by default and add "Close task"

The button label should change depending on the WI type,

i.e Objectives should have label as "Close objective" and "Reopen objective" in case the WI is closed

Current	Expected

Make add comment/add reply appear as inputs rather than buttons

Mirroring the "Add reply" state on issuables, the collapsed state of the input should appear as though it was a text box without a hover state and with a text cursor on hover.

Current	Expected
	(but leave as Sans font, given the rich text editor as default)

Once activated, the editor should appear and receive focus.