feat: nixpkgsissue activity log view

[alejandrosame]

feat: nixpkgsissue activity log view

Improve the messy representation of the different pghistory event tables:

• Create a view log_nixpkgsissue in the PostgreSQL backend and introduce an unmanaged model NixpkgsIssueLogView to display a list of changes per timestamp and user.
• Register NixpkgsIssueLogView in the pghistory app to declutter the log reporting from the already crowded shared section.
• Delete the previous activitiy log models and views in favour of the unified view model.

If performance starts being problematic for the log_nixpkgsissue view, we can make it a materialized view and add a trigger to update its contents on shared.nixpkgsissue and it's m2m relationships updates.

NOTE: This PR includes #101

[alejandrosame]

@RaitoBezarius the cascade does nothing because it prevents Django from trying to delete content from a postgres VIEW, which would just yield an error (it's read-only after all).

The data is collected from other auth and pghistory tables so we have to double check there what exactly we want to do on user deletion. There's also the default Django admin table, which registers less finegrained changes made by users.

I'll make an issue to not to forget about looking into this.

[alejandrosame]

Please don't merge yet. Working on some other functionality I noticed a mistake on the way I set the migration file in this PR.

I'll take this out of draft as soon as I push the fix.

[alejandrosame]

The PR is in a mergeable state again.

[alejandrosame]

rebased with latest changes

[alejandrosame]

@RaitoBezarius if you can reconfirm that you are ok with this approach after the rebases, this is ready to be merged.

[fricklerhandwerk]

Towards https://github.com/Nix-Security-WG/nix-security-tracker/issues/169