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.)

app/controllers/application_controller.rb

@@ -5,8 +5,6 @@ class ApplicationController < ActionController::Base
   # For APIs, you may want to use :null_session instead.
   protect_from_forgery with: :exception
 
-  force_ssl if: :https_enabled?
-
   include Localized
   include UserTrackingConcern
   include SessionTrackingConcern
@@ -31,10 +29,6 @@ class ApplicationController < ActionController::Base
 
   private
 
-  def https_enabled?
-    Rails.env.production?
-  end
-
   def store_current_location
     store_location_for(:user, request.url) if request.format == :html
   end

app/controllers/settings/profiles_controller.rb

@@ -25,7 +25,7 @@ class Settings::ProfilesController < Settings::BaseController
         AccountUsernameChange.create!(
           account: @account,
           from_username: @account.username,
-          to_username: params[:account][:username]
+          to_username: params[:account][:username] || ''
         )
       end