Advanced search: Add planner role support to milestones
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Problem to solve
The planner role was introduced in #482733 (closed).
Milestone search is dependent upon two feature_access_level values: issues_access_level and merge_requests_access_level.
Milestone search does not work for some cases for users when they only have merge_requests_access_level set
https://docs.gitlab.com/user/permissions/#project-planning
Proposal
Implement the planner role for milestone search. The Search::Elastic::MilestoneQueryBuilder 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))