[WIP] Some serious refactoring

This is a WIP PR, it is unfinished and would be updated with new commits. But you can already review it :)

The goal of this PR is to apply the [latest naming concept](http://yandex-ui.github.io/stylobate-islands/classnames/) that fits stylobate to the nanoislands.

## Checklist

- [x] Use global `is-nb-disabled` modifier instead of local `…_disabled` ones.
- [x] Use hash iteration for global gap variables.
- [x] Rename global `_hidden` modifier to use namespace: `is-nb-hidden`.

    - [ ] Fix nanoblocks so there won't be hardcoded `_hidden` class in it, so we could use only one global modifier.

- [ ] Rename buttons

    - [x] Rename basic stuff (themes, sizes).
    - [ ] Replace occurences of the old code remaining there.
    - [ ] Refactor the yate templates (they're not DRY now).

- [ ] Rename other things (this list would grow)