refactor: prefer described_class in specs

[mroderick]

This PR is adopting @olleolleolle's suggestion in #2551 (review), to use described_class for better ergonomics.

This is part of a solution for #2554.

---

Use described_class instead of repeating class names in specs.

What changed:

• Rebased onto master, which now enforces RuboCop across the whole codebase via .rubocop_todo.yml (PR chore: enforce RuboCop across the codebase with a todo file #2726). The temporary spec/.rubocop.yml from an earlier version of this branch was removed because it is no longer needed.
• Ran bundle exec rubocop -A --only RSpec/DescribedClass spec/, which corrected 135 offenses across 36 files.
• Fixed a Layout/FirstHashElementIndentation offense introduced by the autocorrect in spec/models/invitation_log_entry_spec.rb.
• Regenerated .rubocop_todo.yml to reflect the reduced offense count.
• Ran the affected specs: 423 examples, 0 failures.

This makes renaming a class under test less cumbersome because the specs no longer hard-code the class name in subject, let, mailer calls, class methods, etc.