BE: Implement GraphQl APIs

Implement logic for the GraphQL API's created in #498392 (closed)

This involves:

1. Custom status widget of work item
2. work item type widget definition on different levels. Here we'll list all available statuses for the given work item type and the top level namespace (group). Depending on the setting on namespace to use custom status or system defined status, we'll chose a different set of statuses in iteration 2
   1. namespace
   2. project
   3. group

See the MR that introduced the mock API for example and the POC that implements these APIs