Introduce ROLE_of: user factory helpers

What does this MR do and why?

Introduces set of factory helpers:

• guest_of
• reporter_of
• developer_of
• maintainer_of
• owner_of

Before

let(:group1) { create(:group) }
let(:group2) { create(:group) }
let(:project1) { create(:project) }
let(:user) { create(:user) }

before do
  group1.add_owner(user)
  group2.add_owner(user)
  project.add_reporter(user)
end

After

let(:group1) { create(:group) }
let(:group2) { create(:group) }
let(:project1) { create(:project) }
let(:user) { create(:user, owner_of: [group1, group2], reporter_of: project) }

It also migrates existing developer_projects and maintainer_projects transient attributes to new flexible attributes.

No user facing changes, only spec improvements.

MR acceptance checklist

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