Merge branch 'master' into glitch-soc/merge-upstream

Conflicts:
- app/controllers/admin/base_controller.rb
  Some refactoring made upstream, no real conflict.
- app/javascript/mastodon/features/compose/components/compose_form.js
  Updated using upstream's code but using maxChars instead of the
  hardcoded length of 500 characters per toot.
- app/javascript/styles/mastodon/components.scss
  Upstream redesigned the onboarding modal. Not sure why we had a
  conflict there.

config/routes.rb

@@ -81,9 +81,7 @@ Rails.application.routes.draw do
   post '/interact/:id', to: 'remote_interaction#create'
 
   get '/explore', to: 'directories#index', as: :explore
-  get '/explore/popular', to: 'directories#index', as: :explore_popular
   get '/explore/:id', to: 'directories#show', as: :explore_hashtag
-  get '/explore/:id/popular', to: 'directories#show', as: :explore_hashtag_popular
 
   namespace :settings do
     resource :profile, only: [:show, :update]