visual alignment of accept/reject friendship buttons

[DJPaul]

When accepting or rejecting a friend request, the order of the buttons is "[ACCEPT] [REJECT]". When clicking either, the other button's visibility is set to 'hidden'. On completion of the AJAX request, the button click is faded out, the html update, and faded back in again.
For the accept button, it then appears as "[ACCEPTED] ]" (note the white space).

This is because the hidden element is set visibility: hidden. I assume this is to prevent the button "jumping" around as soon as it is pressed. When the result button is faded in, the other button should be set to hidden so that there isn't a big gap. I'll attach a screenshot.

[djpaul]

(In [4250]) When accepting/rejecting friend request, improve appearance of "accepted" button. Fixes #3171