comments: remove dysfunctional comment bubble on compare and file views (issue #84)

Several compare and file views would show a comment bubble when hovering
code lines, even though clicking on this bubble would not do anything.

Instead, make sure that the bubble only appears when the diff block is in a
class 'commentable-diff', which can be set from the appropriate places
(changeset and pullrequest views) where a click handler is also attached.