Complete refactor of Gemfile and upgraded gems (#2553)

test/mailers/previews/user_mailer_preview.rb

@@ -2,6 +2,7 @@
 
 class UserMailerPreview < ActionMailer::Preview
   def initialize(_params)
+    super
     @logo = "https://raw.githubusercontent.com/bigbluebutton/greenlight/master/app/assets/images/logo_with_text.png"
     @color = "#467fcf"
   end
@@ -11,7 +12,7 @@ class UserMailerPreview < ActionMailer::Preview
   def password_reset
     user = User.first
     user.reset_token = User.new_token
-    url = "http://example.com" + "/password_resets/" + user.reset_token + "/edit?email=" + user.email
+    url = "http://example.com/password_resets/#{user.reset_token}/edit?email=#{user.email}"
     UserMailer.password_reset(user, url, @logo, @color)
   end
 
@@ -19,7 +20,7 @@ class UserMailerPreview < ActionMailer::Preview
   # http://localhost:3000/rails/mailers/user_mailer/verify_email
   def verify_email
     user = User.first
-    url = "http://example.com" + "/u/verify/confirm/" + user.uid
+    url = "http://example.com/u/verify/confirm/#{user.uid}"
     UserMailer.verify_email(user, url, @logo, @color)
   end