Improve user assignment UI in roles edit page

[DonOmalVindula]

Purpose

$subject

Related Issues

• N/A

Related PRs

• N/A

Checklist

• e2e cypress tests locally verified. (for internal contributers)
• Manual test round performed and verified.
• UX/UI review done on the final implementation.
• Documentation provided. (Add links if there are any)
• Relevant backend changes deployed and verified
• Unit tests provided. (Add links if there are any)
• Integration tests provided. (Add links if there are any)

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines
• Ran FindSecurityBugs plugin and verified report
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.79%. Comparing base (ee459d3) to head (8e71bf6).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7993   +/-   ##
=======================================
  Coverage   41.79%   41.79%           
=======================================
  Files          42       42           
  Lines         938      938           
  Branches      238      217   -21     
=======================================
  Hits          392      392           
  Misses        546      546           

Flag	Coverage Δ
@wso2is/core	41.79% <ø> (ø)

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[wso2-jenkins-bot]

⚠️ No Changeset found

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go.

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