Remove possibility to add organisation managers by username

[matleppa]

Closes #3437

Temporary quick and dirty correction in Organization manager handling.

Changes

• show only managers' email address, not username
• add manager only with email address, not with username

Developer checklist

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• Alternative solutions were compared/discussed before writing code
  • trade-offs with this solution are considered acceptable
• Code in this PR adheres to the project style guide
• This pull request does not decrease project test coverage
• If the code changes existing database collection(s), migration has been written
• If UI texts are added or changed, all texts are internationalized

Reviewer checklist

Reviewed by: @username1

This list is to be completed by the pull request reviewer:

• Code works as described/expected
• Code seems to be error free
  • no browser console errors visible
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  • passes CI build
• Code is written in a way that promotes maintainability
  • easy to understand
  • well organized
  • follows project coding standards and conventions
  • well documented