Fix pointer cursor not shown on avatars

When the cursor is on an avatar wrapper the cursor is shown as a pointer
to inform the user that it can be clicked (which will either show the
contacts menu or insert a mention, depending on the case); the cursor
must be explicitly defined for the "img" element that shows the avatar
too, or otherwise the default cursor would be shown.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>