Add `prefix` helper method to SettingsRepositoryInterface

Just a random syntactical idea. Instead of:

``` php
// constructor
$this->settings = $settings;

// body
$this->settings->get('flarum-auth-facebook.app_id');
$this->settings->get('flarum-auth-facebook.app_secret');
```

We could use:

``` php
// constructor
$this->settings = $settings->withPrefix('flarum-auth-facebook');

// body
$this->settings->get('app_id');
$this->settings->get('app_secret');
```

Also, for extension prefixes, we should probably make the best practice `vendor.package`, i.e. `flarum.auth-facebook` instead of `flarum-auth-facebook`.

Read SettingsRepositoryInterface first and locate its implementations; the issue names no files or tests. Evaluate the proposed withPrefix API and the vendor.package naming convention, then identify the affected tests and confirm the intended prefix behavior before considering the work complete.

Written by the indexing model from the issue text.