Improve alias system

This pull request applies various changes to the alias system:
- Changed the moment an automatic alias is build based on the page title. That way, it's easier to set a custom alias.
- The build of an alias now works in multilingual systems. The alias is build based on the english page title or in case that language is not installed based on the default language.
- The alias build function now properly handles all not allowed characters. It's guaranteed that the builded alias is valid now.
- Renamed `URL-Alias` to `Abkürzung`. That's a more understandable description.
- Various minor changes including indentation fixes