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API: Allow to set or update a merge-request's milestone

_Originally opened at !2107 by @ skakirill._

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See merge request !2755

Add merge_requests/:merge_request_id/closes_issues

_Originally opened at !2660 by @Schniz._

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Added `/api/v3/projects/:id/merge_requests/:merge_request_id/closes_issues` route in the API for getting the issues that should be closed when a merge request merged successfully

See merge request !2779

Added some documentation for it

Added to changelog

Added curl example and an attribute table

Moved the api route definition from "lib/api/issues.rb" to "lib/api/merge_requests.rb"

Fixed the attributes and changed the documentation to be at "merge_requests.md" too

Changed generic titles to more specific titles

added an underscore

added tests. it depends on a newer version of gitlab-test project
I'm doing a  since I need to add a branch to the `gitlab-test` repo

removed the before

using 'iid' instead of 'id' in the description to reference the issues. that makes the tests pass

Removed the 'closes-issues' key from test_env. so it should pass the tests

Moved the two initializations to the describe block

Changed the changelog

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# Conflicts:
#	db/schema.rb