Seterra extra per Sinskiy

GitHub: https://github.com/Sinskiy/seterraextra/

This extensions adds following features for Seterra:
- No corner flag. When enabled, flag in the top right corner of the quiz is invisible

- No label, dark label. When no label is enabled, you won't have a label with the name and the flag of the country you need to click. Dark label makes the label dark

- Label opacity

- Dark mode.

- Dark quiz header. When enabled, the header of the current quiz (that thing at the top of the quiz that displays the timer and stuff) becomes dark

- Remove names boldness. Makes the name (typically of a country) in the header less bold

- Low PC specs version. The extension checks whether there has been an update on page less often