Fix tasks related members input

[joanagmaia]

Changes proposed ✍️

• Added missing label property to model value both in Related members and assignees
• Refactor fetchMembers so that it can fetch for a limited set of users and users that are selected but are not included on that list
• Disable inputs and prevent showing empty tags when options are still loading

Relates to #261

Checklist ✅

• Label appropriately with Feature, Enhancement, or Bug.
• Tests are passing.
• New backend functionality has been unit-tested.
• Environment variables have been updated:
  • Local frontend configuration: frontend/.env.dist.local, frontend/.env.dist.composed.
  • Local backend: backend/.env.dist.local, backend/.env.dist.composed.
  • Configuration docs have been updated.
  • Team members only: update environment variables in override, staging and production env. files and trigger update config script.
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.
• All changes have been tested in a staging site.
• All changes are working locally running crowd.dev's Docker local environment.

[joanreyero]

LGTM!