Code review comment for lp://staging/~bjornt/launchpad/form-overlay-load-external

Change all overlays that load the content using ++form++ to use the new
loadFormContentAndRender() method.

I've updated all callsites I could find, which meant I could remove a
lot of custom io callback handling.

I also updated the milestone overlay to use showError() for its error
handling, since it would be hard to make it use
loadFormContentAndRender(). It should use showError() anyway to be
consistent with other overlays.

loadFormContentAndRender() was added to lazr-js in r119.

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Björn Tillenius | https://launchpad.net/~bjornt