Add ability to disable login and mark accounts as memorial (#5615)

app/controllers/admin/accounts_controller.rb

app/controllers/admin/suspensions_controller.rb

app/javascript/styles/mastodon/landing_strip.scss

app/mailers/notification_mailer.rb

app/mailers/user_mailer.rb

app/models/account.rb

app/models/user.rb

app/services/suspend_account_service.rb

app/views/accounts/_header.html.haml

app/views/accounts/show.html.haml

app/views/admin/accounts/show.html.haml

app/workers/admin/suspension_worker.rb

config/locales/en.yml

config/routes.rb

db/migrate/20171107143332_add_memorial_to_accounts.rb

db/migrate/20171107143624_add_disabled_to_users.rb

db/schema.rb