Migrate v2.4 installations to v3.x

We need a method of migrating users from 2.4 to 3.x.

Things to consider:
- [x] Migrate settings
- [ ] Migrate resources
- [ ] Migrate subscriptions / subscribers
- [ ] Migrate API Keys

AOB (please add to this list)

---

As a comment on the latter, once #183 is merged, we'll need to programmatically migrate users 2.4 API Keys to 3.x using something like the following

```php
$v2ApiKey = $v2User->api_key;

$v3User->tokens()->create([
'name' => 'v2.4 API Key',
'token' => hash('sha256', $v2ApiKey),
'abilities' => ['*'],
'expires_at' => now()->addDays(90),
]);
```

@jbrooksuk We need to decide on the `abilities` and `expires_at`. IMO it makes sense to throw something into the upgrade documentation that old 2.4 API Keys will continue to work for up to 90 days. But if they want to use the API after that, then they'll need to generate a new API Token using the 3.x dashboard.