Advanced search: Add planner role support to merge requests

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Problem to solve

The planner role was introduced in #482733 (closed).

Merge request search is dependent upon two things: project visibility_level and merge_requests_access_level.

https://docs.gitlab.com/user/permissions/#merge-requests

Merge request search does not work for planner roles in all cases

Proposal

Implement the planner role for Merge request search. The Search::Elastic::MergeRequestQueryBuilder class is where the Elasticsearch query is built. This is where the bug lies, somehow the feature access level settings are not granting planner when they should be

We could also switch over to using by_search_level_and_membership from by_project_authorization during this time (since the latter method is deprecated).

note: Update role and permissions docs (search was introduced in !189550 (merged))