OONI Run v2 refactor

[hellais]

Align OONI Run v2 to the original specification.

Includes:

• Switch to using postgresql for supporting OONI Run v2 descriptors
• Always returning the full models when fetching and listing
• Use SQLAlchemy with alembic to handle database migrations
• Upgrade to SQLAlchemy 2.x so that we can benefit from proper typing support
• Fix some bugs in the existing implementation

[hellais]

@DecFox confirmed that he has reviewed this branch until https://github.com/ooni/backend/pull/805/commits/f0c4bfcfd46085f72b337fdfabfe152702785993. I am going to proceed with landing this.

@DecFox if you have some comments about the rest of the review feel free to drop another comment here so I can address it as future work.

[hellais]

I will manually squash the git history to make it easier to look at the changeset later.

[hellais]

@DecFox I rewrote the git history so it's easier to tell what's going on. The last commit you have viewed as all been squashed into https://github.com/ooni/backend/commit/d2aa0af7f3e36385831868363fefc710de49b323, so all the new stuff is just this commit: https://github.com/ooni/backend/commit/c3f522530b2151aebb8fc6ec2363143f05364098.

I am closing this PR as merged.