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

Conflicts:
- app/views/admin/pending_accounts/index.html.haml
  No real conflict, upstream changes too close to glitch-specific
  theming system changes.
- config/navigation.rb
  Upstream redesigned the settings navigation, took those changes
  and re-inserted the flavours menu.

db/migrate/20190409054914_create_user_invite_requests.rb

@@ -0,0 +1,10 @@
+class CreateUserInviteRequests < ActiveRecord::Migration[5.2]
+  def change
+    create_table :user_invite_requests do |t|
+      t.belongs_to :user, foreign_key: { on_delete: :cascade }
+      t.text :text
+
+      t.timestamps
+    end
+  end
+end