Add filter by pipeline status on "Merge requests" page
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### Problem to solve
To review MR's I want to filter only those that have successful pipeline status
### Intended users
Developers who do review
### Proposal
Single User Experience
Open `Merge requests` page. Find search input. Enter `pipeline status` (like `Author`/`Assignee`/`Approver`/etc right now) `is`/`is not` condition, value - should be all available statuses of pipeline (`failed`/`passed`/`cancelled`/etc)
### Permissions and Security
Same permissions like in any search filter
### Documentation
Would need extra option here https://docs.gitlab.com/ee/user/search/#filtering-issue-and-merge-request-lists
### Availability & Testing
How might it affect the quality of the product?
it eases code review for end users
What additional test coverage or changes to tests will be needed?
1) test that you can filter by pipeline
2) test that there is no filter is pipeline doesn't exist
3) test that you can't put arbitrary data to search filter
Will it require cross-browser testing?
yes
Type of tests:
* Unit test changes
* Integration test changes
* End-to-end test change
### What does success look like, and how can we measure that?
success metrics - user will filter MR's faster
acceptance criteria - if pipeline doesn't exists on repo you don't have filter. If does then you can filter MR's by pipeline status
### What is the type of buyer?
Core or starter
### Links / references
no links
issue