Allow for specification of email sender (#311)

* <Allow for configuration of email sender>

* <Configure for localization>

* <Added a change in verify_mail>

app/mailers/user_mailer.rb

@@ -17,11 +17,11 @@
 # with BigBlueButton; if not, see <http://www.gnu.org/licenses/>.
 
 class UserMailer < ApplicationMailer
-  default from: 'notifications@example.com'
+  default from: Rails.configuration.email_sender
 
   def verify_email(user, url)
     @user = user
     @url = url
-    mail(to: @user.email, subject: 'Welcome to BigBlueButton!')
+    mail(to: @user.email, subject: t('landing.welcome'))
   end
 end