mgr/dashboard: Add unique validator

mgr/dashboard: Add unique validator

Relocate an already existing async validator into a separate validator that can be reused by every other form. This validator is useful to check immediately after typing if an entered value, e.g. username, already exists.

The API request will be triggered after a delay of 500ms (can be modified). During this delay, every keystroke will reset the timer, so the REST API is not flooded with request.

Signed-off-by: Volker Theile <vtheile@suse.com>