Regenerate DAO files using civix (#548)

* Regenerate DAO files using civix

CiviVolunteer originally used a hacked version of setup.sh to generate DAO files and SQL from the xml.
That's no longer necessary now, with the command: civix generate:entity-boilerplate
Just had to reorganize the files a little.

* Switch installer to use generated auto_install.sql and update volunteer_need.created column

Switched to the auto-install and auto-uninstall sql files, which required fixing the default value
of the created column. Also updated the BAO and added an upgrade step to fix the column.

This required a core version compatibility bump to 5.28 which requires the mysql version which
supports multiple CURRENT_TIMESTAMP defaults in a single table.

CRM/Volunteer/BAO/Need.php

@@ -60,9 +60,6 @@ function __construct() {
   static function &create($params) {
     // these metadata fields are managed; don't accept them as params
     unset($params['created'], $params['last_updated']);
-    if (empty($params['id'])) {
-      $params['created'] = CRM_Utils_Date::currentDBDate();
-    }
 
     $need = new CRM_Volunteer_BAO_Need();
     $need->copyValues($params);