Remove time inefficient loop in not_blocked_items

[KevinMind]

Relates to: mozilla/addons#15170

Description

Removes time inefficient loop in get_not_blocked_items by filtering from a once created set instead of an inlined list

Context

This patch adds a slow test 20 seconds that can be isolated from the rest of our tests in a follow up PR

Testing

CI passes, this PR will be verified on staging as well.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[willdurand]

Could you squash your commits to get a single commit in this PR? I know GitHub will do it eventually but it'd be easier to navigate the changes locally.

[KevinMind]

Ok @willdurand let's do it your way. I have things to say about your feedback but PR comments is not the place, especially when the goal here is to "unbreak prod on the short term"

[willdurand]

r+, thanks