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

Conflicts:
- `app/controllers/concerns/sign_in_token_authentication_concern.rb`:
  Conflict due to glitch-soc's theming system.
  Ported upstream changes.
- `app/controllers/concerns/two_factor_authentication_concern.rb`:
  Conflict due to glitch-soc's theming system.
  Ported upstream changes.

db/migrate/20200620164023_add_fixed_lowercase_index_to_accounts.rb

@@ -0,0 +1,15 @@
+class AddFixedLowercaseIndexToAccounts < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    rename_index :accounts, 'index_accounts_on_username_and_domain_lower', 'old_index_accounts_on_username_and_domain_lower' unless index_name_exists?(:accounts, 'old_index_accounts_on_username_and_domain_lower')
+    add_index :accounts, "lower (username), COALESCE(lower(domain), '')", name: 'index_accounts_on_username_and_domain_lower', unique: true, algorithm: :concurrently
+    remove_index :accounts, name: 'old_index_accounts_on_username_and_domain_lower'
+  end
+
+  def down
+    add_index :accounts, 'lower (username), lower(domain)', name: 'old_index_accounts_on_username_and_domain_lower', unique: true, algorithm: :concurrently
+    remove_index :accounts, name: 'index_accounts_on_username_and_domain_lower'
+    rename_index :accounts, 'old_index_accounts_on_username_and_domain_lower', 'index_accounts_on_username_and_domain_lower'
+  end
+end