Require jquery.ui.position

Fixes the link-to-this popup box rendering at the bottom left of the
page.

Introduced by f91d66d42f517f778cac130466b7cffc7fd8b085 as we rely on
jQuery UI's position method http://api.jqueryui.com/position
Improve positioning of link-to-this popup

Current positioning forces it to overlap the sidebar, which:

- Looks messy due to the opacity
- Covers potentially useful information
- Puts the close icon further away if the box was opened in error

This commit positions it to the left of the link-to-this anchor icon,
which seems to conveniently fit in to the whitespace left by the signing
off of the correspondence.
Merge branch 'issues/1405-link-to-this-box' into rails-3-develop