Introduce membership attributes for project and group factories

What does this MR do and why?

Introduces helper transient attributes for project and group factories. Main changes are in !149259 (diffs) and !149259 (diffs) everything else is just grep syntax update.

Before

let_it_be(:developer) { create(:user) }
let_it_be(:project) { create(:project) }
let_it_be(:group) { create(:group) }

before_all do
  project.add_developer(developer)
  group.add_developer(developer)
end

After

let_it_be(:developer) { create(:user) }
let_it_be(:project) { create(:project, developers: developer) }
let_it_be(:group) { create(:group, developers: developer) }

No user facing changes, only spec improvements.

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