#41 - Implement add assignment modal

[NeemZ16]

Proposed changes

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Added global styling for modals, created modal component in shared/layouts/modal.tsx, and implemented add assignment modal.

Closes #207

Types of changes

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What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of
them, don't hesitate to ask. This is simply a reminder of what we are going to look for before merging your code.

• My changeset covers only what is described above (no extraneous changes)
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

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If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you
did and what alternatives you considered, etc...

[dvcabiya]

Looks great