Update to CodeIgniter 3.1.10

[Addono]

Should be easy, as the API didn't really change much, see:
https://www.codeigniter.com/user_guide/installation/upgrade_3110.html

Changes seems minor, mostly some small bug fixes.

Todo

1. Replace the system folder with the source of CI 3.1.10
2. Just to be completely sure: Check for any usages of is_countable() - probably there aren't any.
3. Install the custom languages from here by copying the content into the system folder.
4. Briefly test if everything is still working
5. PR time 🎉