Merge lp://staging/~gary/launchpad/bug713392 into lp://staging/launchpad

[r=gmb][bug=713392] make the +structural-subscriptions UI correctly show the effect of an event filter setting.

This branch fixes bug 713392 by making the +structural-subscriptions UI (e.g., https://bugs.launchpad.dev/~no-priv/+structural-subscriptions) correctly show the effect of an event filter setting.

While I find the +structural-subscriptions page difficult or even impossible to find via Launchpad's navigation, and we hope to replace it with another UI, it is available now to everyone, so I thought there was value in having it be correct.

I added code and tests for the situation when a structural subscription's bug notification level is NOTHING. This is not expected to happen, ever, so that was probably overkill.

Lint is happy.

To run tests, run ./bin/test -vvtTestBugSubscriptionFilterView

To QA, log in as a user that has malone.advanced-subscriptions.enabled true (on production, that means you are a member of the malone alpha team; on dev, on https://launchpad.dev/+feature-rules you can set "malone.advanced-subscriptions.enabled default 100 on" as <email address hidden>:test). Then make sure that user is structurally subscribed to some project. Then go to the +structural-subscriptions page for the logged in user (replace "no-user" with the proper id in "https://bugs.launchpad.dev/~no-priv/+structural-subscriptions") and add a filter with a limited event subscription (see the bottom of the add page, and choose "a bug is fixed or re-opened" or "Any change is made to a bug other than a new comment being added."). Then click add, and verify that what you did is reflected in the +structural-subscriptions page.