The introduction of General Comments added a button to the Review Dialog

and to the review request's action bar saying "Add Comment." This label

doesn't make clear that it's a General Comment being added, and could

easily be confused as the way one is supposed to do code review

(similarly to how some people have been confused in the past trying to

put their review in the Header field).
This change renames these both to "Add General Comment," helping to

clarify just a bit. It's a bit longer, but there's plenty of room on the

review request's action bar and in the Review Dialog for this, and it

should hold us over until the new review UI is ready.
It also moves the button in the Review Dialog over to the left of the

dialog, separating it from buttons that impact the overall

review/dialog.

Tested that both of these buttons still work.
Tested the Review Dialog's button placement in desktop and mobile modes.