Move pipelines widget into merge widget inline with the pipeline merge check

Proposal

The pipeline widget on a Merge Request currently is separate from the merge widget where the pipeline check exists. The proposal is to combine the pipeline widget into the merge widget to be more contextual and also to group related information together in order to help the user better understand the state of merge-ability, as well as reduce visual clutter on the MR.

See design exploration here: #415865

We need to account for artifacts, review app, etc. (anything that is produced by the pipeline widget).

Examples:

Review app

Artifacts