This patch enables users who are not using a mouse to take advantage of the 

actions attached to file attachment thumbnails in a review request. When 

a file attachment is tabbed to, the menu appears and the actions in the 

menu can be tabbed back and forth using tab & shift+tab. An action can be

taken using the enter key.
The implementation of the file attachment menu was slightly changed to 

accomodate the new behavior. Rather than using display: none and 

display: block for the file actions container, we use opacity: 0 and

opacity: 1 to show and hide the container. When we use display: none, 

this prevents the children of the container from being tabbed to. In addition,

using padding for the shadow causes the focus ring to be wider than necessary. 

By using margin, we can get a tighter and clearer focus ring with the same 

shadow.

Manual testing:

- using tab & shift+tab to navigate to the file attachments, verifying that the

file actions menu is opened and that each item can be selected using the enter key

and the actions menu is closed once the thumbnail loses focus
Unit tests:

- testing that the hover event is fired when a mouse is hovered over the thumbnail

- testing that the focus event is fired when the thumbnail is tabbed to