This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
- Regular users are not notified of whispers
- Regular users no longer have "stuck" topics in unread
- Additional tracking for staff highest post number
- Remove a bunch of unused columns in topics table