feat: add nexcloud contacts app requirement on nextcloud plugin

Nextcloud plugin can ask user to choose addressbook to let automatically
save the recipients. But it is dependent on
https://github.com/nextcloud/contacts/ to be installed at NC server.
This commit add the check before showing the option to the user.

plugins/nextcloud/README.md

@@ -2,7 +2,7 @@
 
 ## Nextcloud Addressbook for recipients
 
-This plugin can let user to choose which nextcloud addressbook to use save recipients. This is opt-in feature (enabled by admin). After admin enable this, user will find a dropdown in his/her SnappyMail's `Contacts` section, containing all his/her addressbook.
+If server has [Nextcloud Contacts App](https://github.com/nextcloud/contacts/) setup, this plugin can let user to choose which nextcloud addressbook to use save recipients. This is opt-in feature (enabled by admin). After admin enable this, user will find a dropdown in his/her SnappyMail's `Contacts` section, containing all his/her addressbook.
 
 ### Admin settings
 

plugins/nextcloud/index.php

@@ -62,7 +62,7 @@ public function Init() : void
 			$this->addHook('smtp.before-login', 'beforeLogin');
 			$this->addHook('sieve.before-login', 'beforeLogin');
 
-			if ($this->Config()->Get('plugin', self::ENABLE_NC_ADDRESSBOOK_KEY, self::ENABLE_NC_ADDRESSBOOK_DEFAULT_VALUE)) {
+			if ($this->isNcContatcsAppSetup() && $this->Config()->Get('plugin', self::ENABLE_NC_ADDRESSBOOK_KEY, self::ENABLE_NC_ADDRESSBOOK_DEFAULT_VALUE)) {
 				$this->addJs('js/nextcloudAddressbook.js');
 				$this->addJsonHook('NextcloudGetAddressBooks', 'GetAddressBooks');
 				$this->addJsonHook('NextcloudUpdateAddressBook', 'UpdateAddressBook');
@@ -556,6 +556,10 @@ private function UserSettings() : \RainLoop\Settings
 	{
 		return $this->SettingsProvider()->Load($this->Account());
 	}
+
+	private function isNcContatcsAppSetup() : bool {
+		return \OC::$server->getAppManager()->isEnabledForUser('contacts');
+	}
 }
 
 class AddressBook