[META] Use the GraphQL API for the Merge Request Widget

The Merge Request widget currently needs all of this information: https://gitlab.com/gitlab-org/gitlab-ce/snippets/1716388

In order to use GraphQL for that we'll need to address several issues:

• Standardized way to request abilities for a resource: https://gitlab.com/gitlab-org/gitlab-ce/issues/47695
• Standardized way to expose web-urls
• Paginated relations (collections) with batch loading, id-based pagination poc for Pipelines: gitlab-ce#48469
• Add mutations for Merging, MWPS, Approving, WIP. Poc: https://gitlab.com/gitlab-org/gitlab-ce/issues/48719

Follow up from https://gitlab.com/gitlab-org/gitlab-ce/issues/34754 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19008)