Implement a straight forward way to query object visibility

[MythicManiac]

This PR introduces the concept of visibility flags which can be attached to objects and provides a standard interface for querying objects with visibility rules in mind.

The visibility-aware query methods are intentionally kept stupid simple, as the assumption is that the visibility flags will later on be tied to a mechanism which updates them when the underlying object state changes. In other words, the flags which are queried against serve as a sort of cache, while the actual complexities of the visibility rules have to be implemented elsewhere. Ultimately this lets us use far simpler and easier to maintain queries compared to representing all visibility rules on each query site directly.

Some open questions which I deemed out of scope for this PR:

• What is the best way to propagate visibility based querying through relations? For example, if we have a Package model object and want to query it's versions field with some specific visibility constraint, does that work out of the box via the related manager?
  • How about filtering the parent objects based on child visibility? So for example, fetching all packages with at least one version with a specific visibility. Can we have query utilities which streamline this & avoid varying different implementations on different query sites?
  • Essentially the question is how can we best avoid deviating from the intended framework by accident. Current suspicion is that some form of custom query nodes might work nicely.
• Context scoping. This PR intentionally ignores scoping context entirely (e.g. active community) and instead assumes the query caller is taking them into account. Perhaps it'd be worth exploring something similar to this PR but focused on the scoping side.

TODO (This PR):

• [x] Data migration populating the visibility field for existing DB rows
  • Delegated to #1005 as deployment timing has to be staggered
• [x] Tests for the query helpers

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (37cca3a) 92.82% compared to head (ae1f55d) 92.87%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1004 +/- ## ========================================== + Coverage 92.82% 92.87% +0.04% ========================================== Files 309 312 +3 Lines 8898 8956 +58 Branches 786 794 +8 ========================================== + Hits 8260 8318 +58 Misses 530 530 Partials 108 108 ```

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