* Add `from_account` to notifications API

this adds the ability to filter notifications by the account they
originated from

* passing a non-existent user should cause none to be returned

* Fix codeclimate warnings

* fix more codeclimate warnings

* make requested changes:

* use account id instead of user@domain
* name the param `account_id` instead of `from_account`

* Don't use `return` in a lambda