MR Widget Refactor Part 15

Part 15 of the MR widget refactor work.

Previous MRs, all merged into mr-widget-redesign-review

• !9253 (merged)

• !9555 (merged)

• !9579 (merged)

• !9630 (merged)

• !9877 (merged)

• !9963 (merged)

• !10006 (merged)

• !10127 (merged)

• !10219 (merged)

• !10354 (merged)

• !10377 (merged)

• !10423 (merged)

• !10504 (merged)

• !10543 (merged)

• Drop ci_status endpoint

• Drop pipeline_status extra request

• Poll on merge_check and update view

• Validate json schema instead using the serializer

• Move basic check to Entity level

• Use Basic MR serializer for TimeTracker

• Check permission on conflict_resolution_path instead returning 2 fields

• Move mr_assign_issues_link data to serializer and drop old helpers

• MAKE BUILD PASS

@oswaldo - working on Pipelines:

• https://gitlab.com/gitlab-org/gitlab-ce/builds/14354276 [fixed]
• https://gitlab.com/gitlab-org/gitlab-ce/builds/14354286 [fixed]
• https://gitlab.com/gitlab-org/gitlab-ce/builds/14358540 [fixed]

This MR's target branch is also mr-widget-redesign-review. We are doing this because of incremental reviews. This can be merged into review branch whenever reviewer thinks it's OK.

Two minor things to consider:

• After clicking merge, even if we return success status on the merge action, errors may happen while processing the merge on background. When it happens, we update MergeRequest#merge_error, hence we should add a special case (right after checking state == 'merged') looking for merge_error, presenting it on mr_widget_failed_to_merge.js.

• We should check if a project/namespace was removed and show a (removed):target-branch-name - It was caught by integration tests and currently we're not handling this correctly.

Related with #25424 (closed)