Add abilityto add oneself to lists (#12271)

* Add ability to add oneself to lists

* Change search results to include oneself when searching through followers

* Mark follow relation as optional in ListAccount

db/migrate/20191031163205_change_list_account_follow_nullable.rb

@@ -0,0 +1,5 @@
+class ChangeListAccountFollowNullable < ActiveRecord::Migration[5.1]
+  def change
+    change_column_null :list_accounts, :follow_id, true
+  end
+end