Fix rubocop issues, introduce usage of frozen literal to improve performance

.rubocop.yml

Gemfile

app/channels/application_cable/channel.rb

app/channels/application_cable/connection.rb

app/channels/hashtag_channel.rb

app/channels/public_channel.rb

app/channels/timeline_channel.rb

app/controllers/about_controller.rb

app/controllers/accounts_controller.rb

app/controllers/api/salmon_controller.rb

app/controllers/api/subscriptions_controller.rb

app/controllers/api/v1/accounts_controller.rb

app/controllers/api/v1/apps_controller.rb

app/controllers/api/v1/follows_controller.rb

app/controllers/api/v1/media_controller.rb

app/controllers/api/v1/statuses_controller.rb

app/controllers/api/v1/timelines_controller.rb

app/controllers/api_controller.rb

app/controllers/application_controller.rb

app/controllers/auth/confirmations_controller.rb