Backlinks for tasks with block IDs should link straight to the task's line

[claremacrae]

I realised this from the discussion #1150 (comment).

Whilst I'm not keen to get in to automatically adding block ids to task lines, if the user has added one, we should certainly takee advantage of it.

The block-link is already saved inside Task objects, so it should be relatively easy to do.

[claremacrae]

This is no longer relevant, as all backlinks now jump directly to the actual line.
Closing.