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.)

Gemfile

@@ -6,7 +6,7 @@ ruby '>= 2.4.0', '< 2.7.0'
 gem 'pkg-config', '~> 1.3'
 
 gem 'puma', '~> 5.0'
-gem 'rails', '~> 5.2.3'
+gem 'rails', '6.0.3.4'
 gem 'thor', '~> 0.20'
 
 gem 'hamlit-rails', '~> 0.2'
@@ -59,7 +59,7 @@ gem 'kaminari', '~> 1.1'
 gem 'link_header', '~> 0.0'
 gem 'mime-types', '~> 3.2', require: 'mime/types/columnar'
 gem 'nokogiri', '~> 1.10'
-gem 'nsa', '~> 0.2'
+# gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.7'
 gem 'ostatus2', '~> 2.0'
 gem 'ox', '~> 2.10'
@@ -68,7 +68,7 @@ gem 'pundit', '~> 2.0'
 gem 'premailer-rails'
 gem 'rack-attack', '~> 6.0'
 gem 'rack-cors', '~> 1.0', require: 'rack/cors'
-gem 'rails-i18n', '~> 5.1'
+gem 'rails-i18n', '~> 6.0.0'
 gem 'rails-settings-cached', '~> 0.6'
 gem 'redis', '~> 4.2', require: ['redis', 'redis/connection/hiredis']
 gem 'mario-redis-lock', '~> 1.2', require: 'redis_lock'
@@ -127,6 +127,7 @@ group :development do
   gem 'better_errors', '~> 2.5'
   gem 'binding_of_caller', '~> 0.7'
   gem 'bullet', '~> 6.0'
+  gem 'listen'
   gem 'letter_opener', '~> 1.7'
   gem 'letter_opener_web', '~> 1.3'
   gem 'memory_profiler'
@@ -146,7 +147,6 @@ end
 
 group :production do
   gem 'lograge', '~> 0.11'
-  gem 'redis-rails', '~> 5.0'
 end
 
 gem 'concurrent-ruby', require: false