* Dropdown - Allow to disable Popper.js style

* Update dropdown.js

* Update dropdown.html

* copy changes

Less on 2 symbols

Also, refactor ESLint and stylelint config files; now everything will work from our root dir.

This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.