(DX) Civi::contactSettings - Add a facade for working with the logged-in user's settings

(DX) Civi::contactSettings - Add a facade for working with the logged-in user's settings

Before
------

To read/write a setting for the logged-in user, you need a snippet like this:

```php
$cid = CRM_Core_Session::getLoggedInContactID();
$myFilter = Civi::service('settings_manager')
  ->getBagByContact(NULL, $cid)
  ->get('activity_tab_filter');
```

After
-----

To read/write a setting for the logged-in user, you can use a snippet like this:

```php
$myFilter = Civi::contactSettings()->get('activity_tab_filter');
```

Technical Details
-----------------

* A convenience function like this is liable to be used in lazy circumstances where
  the developer is unlikely to check their conditions carefully. Therefore, the
  errors are reported as exceptions so that mistakes are easiliy revealed.
* This is technically a small contract change to `SettingsManager::getBagByContact()`
  because it now interprets `$contactId===NULL` as meaning "the logged-in user".
  However, I don't think NULL was a sensible value before, and this interface
  isn't widely known/used.