Add project import level to namespace settings

Migration Output:

bin/rails db:migrate RAILS_ENV=development
main: == 20220705145349 AddProjectImportLevelToNamespaceSettings: migrating =========
main: -- add_column(:namespace_settings, :project_import_level, :smallint, {:default=>0, :null=>false})
main:    -> 0.0017s
main: == 20220705145349 AddProjectImportLevelToNamespaceSettings: migrated (0.0022s)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #358750 (closed)