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.

* Add dropright (right placement for our dropdown)

* Add dropleft

* moves drop left arrow to the left

We don't support Opera 12, and Firefox's property was the unprefixed one.