Use the new user updates API

This update includes some user search stuff from Exercises.

It also makes the gem use the user updates API. Saving an OpenStax::Accounts::User pushes the changes to Accounts.

```sh
$ rails g openstax:accounts:schedule
```

will generate a "whenever" schedule to get updates from Accounts. With the gem enabled, activate it with:

```sh
$ whenever -wf
```

Deactivate it with:

```sh
$ whenever -wc
```

(had to deactivate manually, clear doesn't seem to work)