Code review comment for lp:~intellectronica/launchpad/bugmail-ui-fixes

This branch makes the menu link for subscribing to bug mail sensitive to the user's existing subsription. If the user is already subscribed (directly, or via one of the teams he's a member of), the link displays the edit icon and offers to edit subscription. If no subscription exists the link offers to subscribe and displays the add icon.

There are no lint warnings related to my changes (the usual lazr stuff is still there).

To test, run bin/test -vv -t xx-milestone-add-and-edit.txt -t xx-distroseries-index.txt -t xx-productseries-index.txt -t xx-productseries-add-and-edit.txt -t xx-bug-subscriptions.txt -t xx-distribution-bugs-page.txt -t xx-distrorelease-bugs-page.txt -t xx-product-bugs-page.txt > test-output.txt

To play with the UI, go to https://bugs.launchpad.dev/jokosher or https://bugs.launchpad.dev/ubuntu or any of the other structural subscription targets and try to subscribe and unsubscribe.

While working on this I realised that the form for editing structural subscriptions does not behave in the expected way, like all other boomerang forms. It doesn't offer a cancel link, and after submitting it displays the form again, instead of returning to the context. I plan to fix that in a future branch.