Misc plugin improvements

[mariusandra]

Changes

• Sort the repository alphabetically
• Open the "repository" tab if no plugins are installed (and we have access to it)
• Fix issue where the same plugin can't be installed in different organisations (plugins are scoped to organisation)
• Test for the latter [WIP]

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests

[mariusandra]

A bunch of fixes and simplifications that can be looked at and merged. Probably more still to come, but then in other PRs.

Aside: once things quiet down, we needs tests on the frontend here!