Set param attempt_group_search_optimizations or
attempt_project_search_optimizations as
true in Boards::Issues::ListService to force CTE

Include all Issue columns in group clauses

When ordering labels priority if CTE search
optimisation was used we need to include
all Issue columns in group clauses to
prevent a PG::GroupingError.

Add specs for board issues controller

Test index request using search param

Add relative_position as a simple sort

Check board parent before setting search params

If the board parent is a group we need to use
attempt_group_search_optimizations and if it's
a project attempt_project_search_optimizations

Refactor issue_grouping_columns method

Add feature flag board_search_optimization

Include a new feature flag to test performance
changes introduced by enabling CTE optimization
for boards search.
Because of this change we need to modify the
method order_labels_priority in Issuable concern
to prevent a PG::GroupingError when grouping result

Add more specs to board issues ListService