Fix NameError: import Deadline model in deadlineManagement.py

[baobao1355]

The NameError was because the Deadline model was not imported. I fixed by adding from app.models import Deadline to the top of deadlineManagement.py

[MImran2002]

I will approve this PR but the next PR I would require the contributor to create a proper PR description in the format like this:

Issue Description
Fixes issue #<issue_number>

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Moreover, you did well with structuring the imports according to this repo the standard is to import in this order general/system import > logic > model

[baobao1355]

Thank you! I will do that in the future