Add a merge_requests/:iid/merge_to_ref API which make
use of the groundwork to write merge results into
refs/merge-requests/:iid/merge.

This reverts commit 7981c029, reversing
changes made to 9202bbd1.

Use shared examples for issues and merge requests
rather than a loop creating common specs.

For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.

Add documentation for issues and MRs. Add shared tests
for both bulk updating issues and MRs.

Issues
https://gitlab.com/gitlab-org/gitlab-ce/issues/47149
https://gitlab.com/gitlab-org/gitlab-ce/issues/56014

Replace all '9koXpg98eAheJpvBs5tK' occurrences with
'<your_access_token>' in API docs.

Documentation mixed up 406 and 405 error codes, this corrects that

Also elaborated on the possible ways you can receive a 405 error code

See spec test: https://gitlab.com/powisj/gitlab-ce/blob/master/spec/requests/api/merge_requests_spec.rb#L848

By using the parameters `?labels=None|Any` the user can filter
issues/mrs from the API that has `none/any` label.

Affected endpoints are:

- /api/issues
- /api/projects/:id/issues
- /api/groups/:id/issues
- /api/merge_requests
- /api/projects/:id/merge_requests
- /api/groups/:id/merge_requests

It resolves #52589 issue

Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG
improved comments in API code
improved spec description

Closes #42445.

"Maintainer" will be freed to be used for #42751

Deprecate corresponding dash versions created-by-me and assigned-to-me