admin panel should use API

Rewrite admin panel to be part of JS application on client side and use rest API.

added refactor label

changed milestone to %v12.0.0

mentioned in issue #184 (closed)

created branch 101-admin-panel-should-use-api

mentioned in merge request !115 (merged)

closed via commit 60dace89

closed via merge request !115 (merged)

mentioned in commit 60dace89