[OPT-4786] I can use assignee/candidate group filter for dashboards

AT:

in the dashboard edit mode, in the add a filter dropdown, there are two new options: "Assignee" and "Candidate Group"
When clicking either option, if the dashboard contains unsaved reports, a modal asks the user to save the dashboard before continuing
When clicking either option, a modal opens containing the same UI as when adding an assignee/candidate group filter to a report, with the following differences:
- At the top of the modal, there is a text clarifying that selected options will be available in the resulting filter and that the filter will operate on flow node level
- There is an "Allow viewer to add filter values" checkbox
In the dashboard view mode, when an assignee/candidate group filter has been added as available filter, the viewer can use these new filters
The UI looks similar to the one we already have for String variables, with the following differences
- The name of the Assignee/Candidate Group is displayed for predefined options
- For custom values, instead of the values typeahead, we use the UsersList component, showing name, email and id of the users

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