Fixes #4673 Improvements in user management screen

[JanhaviAlekar]

fixes #4673

Proposed changes

commit 1: Fixed duplicate "Accept Invitation" button issue
commit 2: Fixed dialog box not closing after deleting an invite
commit 3: Fixed dialog box not closing after leaving a project
commit 4: Added toast error message to indicate that the username is already taken

Types of changes

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

Checklist

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• I have read the CONTRIBUTING doc
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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

[hrishavjha]

LGTM, thanks for the changes 🚀

[amityt]

Please resolve the merge conflicts @JanhaviAlekar