Management command tests

[ddabble]

Depends on #806.

Proposed changes

Added some management command tests (62d75bb). This removes the need for the following checklist item in pull_request_template.md:

• I've run makemigrations, makemessages and compilemessages

Also:

• Added missing migration changes (5ea3b2c)
• Upgraded django-decorator-include (9124f7e)

Review guidance

Checklist

(If any of the points are not relevant, mark them as checked, so that it's easy to see which points you've handled or not)

• I've written tests that fail without these changes (if relevant/possible)
• I've manually tested the website UI with different device layouts
  • Most common is to test with typical screen sizes for mobile (320-425 px), tablet (768 px) and desktop (1024+ px), which can easily be done with your browser's dev tools
• I've manually tested with different users locally
  • This can be e.g. anonymous users (i.e. not being logged in), "normal" non-member users, members of different committees, and superusers
• I've made sure that my code conforms to the code style guides
  • It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you use it as a reference guide / checklist while taking a second look at your code before opening a pull request
• I've attempted to minimize the number of common code smells
  • See the comment for the previous checkbox
• I've added documentation
  • E.g. comments, docstrings, or in the README
• I've added my changes to the "Unreleased" section of the changelog, together with a link to this PR
  • Mainly the changes that are of particular interest to users and/or developers, if any
• I've added a "Deployment notes" section above and labelled the PR with has-deployment-notes
  • ...if anything out of the ordinary should be done when deploying these changes to the server (e.g. adding/removing an environment variable, manually creating/changing some objects, running a management command, etc.)
• I've structured my commits reasonably
  • Some tips on this can be found in "Write Better Commits, Build Better Projects" and in the code style guides
  • Restructuring commits after they've already been committed, can be done using e.g. interactive rebase; see the Git docs on rewriting history