Upgrade to Rails 6

- Added listen gem because rails app:update failed without it
- Removes nsa gem because it requires activesupport < 6
  Oddly, there's a newer version per GitHub that's not on RubyGems
  that raises the requirement to < 7. But, we need a new release
  before we can use that.
- Upgrade rails-i18n to v6, for Rails 6 compatibility
- Remove redis-rails since it requires Rails < 6, but we can just use
  the regular Rails :redis_cache_store that was introduced in 5.2.
- Run `rails app:update`
  - Had to weed through the `config/environments` changes to pick up
    newly added settings, while not losing settings that were
    intentionally configured
  - This generated config/initializers/new_framework_defaults_6_0.rb.
    The goal is to uncomment all of the settings in that file. Once
    they're all uncommented, delete the file and edit
    config/application.rb's load_defaults to be 6.0.
  - force_ssl controller method is deprecated in favor of a Rails
    config setting.
  - bin/setup was generated by Rails
  - Verbose query logging in development is really helpful. It shows
    you what line of code caused a query to execute.
  - Calling establish_connection after worker boot isn't necessary
    with ActiveRecord anymore. ActiveRecord handles it gracefully.
  - Rails generated a migration to add a foreign key for ActiveStorage.
    Gab currently doesn't use ActiveStorage, and instead uses Paperclip
    (since ActiveStorage didn't exist when Mastadon started). So, this
    change isn't relevant to Gab.
- Added ` || ''` in the profiles controller, because I was unable
  to save a change to a newly created profile. (This probably
  explains the 500 error I received on prod Gab recently.)

config/environments/development.rb

@@ -17,7 +17,7 @@ Rails.application.configure do
   if Rails.root.join('tmp/caching-dev.txt').exist?
     config.action_controller.perform_caching = true
 
-    config.cache_store = :redis_store, ENV['REDIS_URL'], REDIS_CACHE_PARAMS
+    config.cache_store = :redis_cache_store, REDIS_CACHE_PARAMS.merge(url: ENV['REDIS_URL'])
 
     config.public_file_server.headers = {
       'Cache-Control' => "public, max-age=#{2.days.to_i}",
@@ -38,6 +38,9 @@ Rails.application.configure do
   config.x.vapid_private_key = vapid_key.private_key
   config.x.vapid_public_key = vapid_key.public_key
 
+  # Store uploaded files on the local file system (see config/storage.yml for options).
+  config.active_storage.service = :local
+
   # Don't care if the mailer can't send.
   config.action_mailer.raise_delivery_errors = false
 
@@ -49,6 +52,9 @@ Rails.application.configure do
   # Raise an error on page load if there are pending migrations.
   config.active_record.migration_error = :page_load
 
+  # Highlight code that triggered database queries in logs.
+  config.active_record.verbose_query_logs = true
+
   # Debug mode disables concatenation and preprocessing of assets.
   # This option may cause significant delays in view rendering with a large
   # number of complex assets.
@@ -76,6 +82,8 @@ Rails.application.configure do
   # Otherwise, use letter_opener, which launches a browser window to view sent mail.
   config.action_mailer.delivery_method = (ENV['HEROKU'] || ENV['VAGRANT'] || ENV['REMOTE_DEV']) ? :letter_opener_web : :letter_opener
 
+  config.file_watcher = ActiveSupport::EventedFileUpdateChecker
+
   config.after_initialize do
     Bullet.enable        = true
     Bullet.bullet_logger = true