* fix the update_version_check.sh Travis CI script

* added 2 more paths to the .gitignore file

* laid the groundwork for the SEO sitemap.xml generation

* unified the naming of various *AvailabilityRegistrar classes, etc.

* save current work in progress at implementing the sitemap generator

* - updated the .gitignore file to ignore sitemap.xml files too

* extended the ModelSaveEvent with the called module name

* added a first rough version of the XML sitemap generation

* updated the changelog

* minor fix

* moved some files around...

* - implemented the sitemap generation capability for the news module
- refactored the SitemapAvailabilityExtension class a little bit to reduce the code duplication

* implemented the sitemap generation capability for the contact module

* implemented the sitemap generation capability for the files module

* minor code refactoring

* implemented the sitemap generation capability for the gallery module

* extended the sitemap generation of the files module

* - added a new switch to the SEO settings to enable or disable the sitemap generation
- added a new switch to the SEO settings to select whether the sitemap should be generation on model save or manually

* added the possibility to manually refresh/generate the SEO sitemap

* do not trigger the page cache invalidation when in developer mode, as the page cache is disabled when the developer mode is engaged

* - minor bug fix for refresh ACP3 installation
- extended the validation of the mailer system settings

* updated the changelog

* improved the changelog

* added some references to related Github issues to the changelog