Add "signed in as" header to some pages (#4523)

app/controllers/application_controller.rb

app/controllers/authorize_follows_controller.rb

app/controllers/remote_follow_controller.rb

app/javascript/styles/basics.scss

app/javascript/styles/containers.scss

app/javascript/styles/forms.scss

app/views/authorize_follows/show.html.haml

app/views/layouts/modal.html.haml

config/application.rb

config/initializers/doorkeeper.rb

config/locales/ar.yml

config/locales/bg.yml

config/locales/ca.yml

config/locales/de.yml

config/locales/en.yml

config/locales/eo.yml

config/locales/es.yml

config/locales/fa.yml

config/locales/fi.yml

config/locales/fr.yml