Enforce instance and namespace level merge method

What does this MR do?

Related to #35266

Closes #1180

Adds the options to set a merge method for new repositories at instance-level and at namespace (group) level. Also allows to enforce such settings by leveraging the cascading settings framework (introduced by !55678 (merged)).

Screenshots

Enforce from Admin Area

Admin Area	Group	Subgroup	Project	User Namespaced Project

Enforce from root group

Admin Area	Group	Subgroup	Project	User Namespaced Project

Enforce from subgroup

Admin Area	Group	Subgroup	Project	User Namespaced Project

No enforcement

Admin Area	Group	Subgroup	Project

Migrations

This MR introduces two new DB migrations.

UP

$ bundle exec rails db:migrate:up VERSION=20220404055410
== 20220404055410 AddMergeMethodCascadingSetting: migrating ===================
-- column_exists?(:namespace_settings, :merge_method)
   -> 0.0040s
-- column_exists?(:namespace_settings, :lock_merge_method)
   -> 0.0026s
-- column_exists?(:application_settings, :merge_method)
   -> 0.0945s
-- column_exists?(:application_settings, :lock_merge_method)
   -> 0.0980s
-- add_column(:namespace_settings, :merge_method, :integer, {:limit=>2, :default=>nil, :null=>true})
   -> 0.0038s
-- add_column(:namespace_settings, :lock_merge_method, :boolean, {:default=>false, :null=>false})
   -> 0.0068s
-- add_column(:application_settings, :merge_method, :integer, {:limit=>2, :default=>0, :null=>false})
   -> 0.0034s
-- add_column(:application_settings, :lock_merge_method, :boolean, {:default=>false, :null=>false})
   -> 0.0020s
== 20220404055410 AddMergeMethodCascadingSetting: migrated (0.2157s) ==========

$ bundle exec rails db:migrate:up VERSION=20220404205154
== 20220404205154 AddMergeRequestsMergeEnabled: migrating =====================
-- add_column(:projects, :merge_requests_merge_enabled, :boolean, {:null=>false, :default=>false})
   -> 0.0060s
== 20220404205154 AddMergeRequestsMergeEnabled: migrated (0.0061s) ============

DOWN

$ bundle exec rails db:migrate:down VERSION=20220404205154
== 20220404205154 AddMergeRequestsMergeEnabled: reverting =====================
-- remove_column(:projects, :merge_requests_merge_enabled)
   -> 0.0112s
== 20220404205154 AddMergeRequestsMergeEnabled: reverted (0.0113s) ============

$ bundle exec rails db:migrate:down VERSION=20220404055410
== 20220404055410 AddMergeMethodCascadingSetting: reverting ===================
-- column_exists?(:namespace_settings, :merge_method)
   -> 0.0047s
-- remove_column(:namespace_settings, :merge_method)
   -> 0.0051s
-- column_exists?(:namespace_settings, :lock_merge_method)
   -> 0.0026s
-- remove_column(:namespace_settings, :lock_merge_method)
   -> 0.0012s
-- column_exists?(:application_settings, :merge_method)
   -> 0.1043s
-- remove_column(:application_settings, :merge_method)
   -> 0.0048s
-- column_exists?(:application_settings, :lock_merge_method)
   -> 0.0980s
-- remove_column(:application_settings, :lock_merge_method)
   -> 0.0020s
== 20220404055410 AddMergeMethodCascadingSetting: reverted (0.2232s) ==========

Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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