Adding is_manager feild

[Sahil590]

Description

Adding is_manager field

Fixes #70 and #72 (issue)
I have added the new field to the model

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• The documentation builds and looks OK (eg. python -m sphinx -b html docs docs/build)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[cc-a]

Implementation looks good but this seems to include the changes from #98.

[Sahil590]

My bad, I was making this change while on that branch and switched to this one with the changes. Just fixed it

[Sahil590]

Added the logic for the send_group_invite view since it was a small change

[cc-a]

LGTM.

[cc-a]

Actually... please add a test for the view function change.

[cc-a]

I'm going to be super picky and say that I'd prefer a linear history for the migrations. Rather than merging you can just delete the migrations added in this PR and makemigrations again and you should end up with a single migration.