HTML e-mails for UserMailer (#6256)

- premailer gem to turn CSS into inline styles automatically
- rework UserMailer templates
- reword UserMailer templates

config/initializers/assets.rb

@@ -8,6 +8,6 @@ Rails.application.config.assets.version = '1.0'
 
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.
-# Rails.application.config.assets.precompile += %w(application_public.js custom.css)
+# Rails.application.config.assets.precompile += %w()
 
 Rails.application.config.assets.initialize_on_precompile = true

config/initializers/premailer_rails.rb

@@ -0,0 +1,6 @@
+require_relative '../../lib/mastodon/premailer_webpack_strategy'
+
+Premailer::Rails.config.merge!(remove_ids: true,
+                               adapter: :nokogiri,
+                               generate_text_part: false,
+                               strategies: [PremailerWebpackStrategy])