api/ui: Its possible to revoke"grant permission" from last user of a workflowitem

[Stezido]

Description

It should not be possible to revoke "grant permission"-permission from the last user. If so the workflowitem is kind of dead. The only way to resolve such issue is to use the root user. The root user shouldn't be used in any case except creating the first user.

Solution

Edit the revoke permission endpoint of the api so revoking the last user isn't possible anymore.
Check project, subproject and workflowitem and disallow it for both api and ui.

[Stezido]

The described business logic was added to the api.
The ui should prevent a user revoking a permission from him/herself. A prop was missing which leads to the bug that a user could revoke a permission from him/herself on workflowitem level. E2E-tests were added on all levels.