Adding Serbian translation (#6133)

app/helpers/settings_helper.rb

app/javascript/mastodon/locales/sr.json

app/views/user_mailer/confirmation_instructions.sr.html.erb

app/views/user_mailer/confirmation_instructions.sr.text.erb

app/views/user_mailer/password_change.sr.html.erb

app/views/user_mailer/password_change.sr.text.erb

...views/user_mailer/reset_password_instructions.sr.html.erb

...views/user_mailer/reset_password_instructions.sr.text.erb

config/application.rb

config/locales/activerecord.sr.yml

config/locales/devise.sr.yml

config/locales/doorkeeper.sr.yml

config/locales/simple_form.sr.yml

config/locales/sr.yml