DEV: Allow custom hosts in development mode

Rails 6 seems to introduce a whitelist of allowed hosts. I personally
use `dev.local` for development and this no longer works.

This introduces a new ENV variable, `DISCOURSE_DEV_HOST`. If present,
it will whitelist that host for development mode.

config/environments/development.rb

@@ -41,6 +41,7 @@ Discourse::Application.configure do
   BetterErrors::Middleware.allow_ip! ENV['TRUSTED_IP'] if ENV['TRUSTED_IP']
 
   config.load_mini_profiler = true
+  config.hosts << ENV['DISCOURSE_DEV_HOST'] if ENV['DISCOURSE_DEV_HOST']
 
   require 'middleware/turbo_dev'
   config.middleware.insert 0, Middleware::TurboDev