5971: added organizations to invitation model

[ArthurMousatov]

Summary

As detailed in Issue #5971, this PR adds organizations to the invitation model. The saving behavior for organizations is slightly different to channels, as there is a separate model for org roles.

I have also omitted tests in this PR, as this is just a model change and does not affect current code (organizations are fully optional in the invitation). I suggest moving testing to the next implementation step (Organization/Invitation viewsets).

References

Direct PR for Issue #5971.

Reviewer guidance

As part of these changes, migrations were run successfully. I tried running make test, but it ran into 905 failures, mostly related to S3Storage errors. No linting was performed, since no frontend work was done.

AI usage

Only basic VS code autocomplete was used.

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