Add missing system setting for profile photo media source (#16515)

### What does it do?
Adds a long-missing setting for specifying user's profile photo media
source. Also, moves up (and tweaks) the associated Lexicon entries.

### Why is it needed?
The logic has been present in the backend to specify a Media Source
other that the default for user profile photos since they were
introduced nearly 10 years ago. However, the system setting was never
added to the build, making it virtually unknown that this functionality
existed.

### How to test
Run `_build/transport.core.php` to install the new setting. Then search
for `photo_profile_source` in the system settings, change its value, and
verify that the Media Source shown in the Media Browser (when
editing/adding a user profile photo) matches that which you specified in
the setting.

### Related issue(s)/PR(s)
Resolves #16510

_build/data/transport.core.system_settings.php

@@ -434,6 +434,15 @@
     'area' => 'manager',
     'editedon' => null,
 ], '', true, true);
+$settings['photo_profile_source'] = $xpdo->newObject(modSystemSetting::class);
+$settings['photo_profile_source']->fromArray([
+  'key' => 'photo_profile_source',
+  'value' => null,
+  'xtype' => 'modx-combo-source',
+  'namespace' => 'core',
+  'area' => 'manager',
+  'editedon' => null,
+], '', true, true);
 $settings['default_per_page'] = $xpdo->newObject(modSystemSetting::class);
 $settings['default_per_page']->fromArray([
   'key' => 'default_per_page',

core/lexicon/en/setting.inc.php

@@ -234,6 +234,9 @@
 $_lang['setting_default_media_source_type'] = 'Default Media Source Type';
 $_lang['setting_default_media_source_type_desc'] = 'The default selected Media Source Type when creating a new Media Source.';
 
+$_lang['setting_photo_profile_source'] = 'User Profile Photo Source';
+$_lang['setting_photo_profile_source_desc'] = 'Specifies the Media Source to use for storing and retrieving profile photos/avatars. If not specified, the default Media Source will be used.';
+
 $_lang['setting_default_template'] = 'Default Template';
 $_lang['setting_default_template_desc'] = 'Select the default Template you wish to use for new Resources. You can still select a different template in the Resource editor, this setting just pre-selects one of your Templates for you.';
 
@@ -504,9 +507,6 @@
 $_lang['setting_proxy_username'] = 'Proxy Username';
 $_lang['setting_proxy_username_desc'] = 'The username to authenticate against with your proxy server.';
 
-$_lang['setting_photo_profile_source'] = 'User photo Media Source';
-$_lang['setting_photo_profile_source_desc'] = 'The Media Source used to store users profiles photos. Defaults to default Media Source.';
-
 $_lang['setting_phpthumb_allow_src_above_docroot'] = 'phpThumb Allow src Above Document Root';
 $_lang['setting_phpthumb_allow_src_above_docroot_desc'] = 'Indicates if the src path is allowed outside the document root. This is useful for multi-context deployments with multiple virtual hosts.';