[BUG] Bug sending `user_count_exceeds_plan_limit_email`

[mattwr18]

Steps to reproduce

• Seed your db, if not already seeded
• Log in as the contact-person@example-organization.org user.
• If you haven't changed anything, the organization's business plan allows 5 users, of which one was created already.
• Create more users until there are more users belonging to the organization than the business plan allows. This can be done in rails console by copy/pasting the user creation from the seeds file, changing the email, or in the UI on the /profile page.
• This will schedule the PostmarkAdapter::Outbound.send_user_count_exceeds_plan_limit_message!(admin, organization) job.
• See the error in the job worker logs:

FAILED (0 prior attempts) with ActionView::Template::Error: no implicit conversion of Array into String

Dev Notes

This occurs because the method user_count_exceeds_plan_limit_email sets the @text instance variable to an array instead of a string:

@text = [subject, I18n.t('adapter.postmark.user_count_exceeds_plan_limit.text', organization_name: organization.name)]