modal: make block (closes #798)

- [x] main idea
- [x] bh templates + tmpl-specs
- [x] specs
- [x] gemini tests
I don't understand what the "runtime" you're talking about? Blocks's `this.params` is 

```js
this.params = objects.extend(this.getDefaultParams(), params);
```

So there shouldn't be any reasons to leave magical `|| 20` part inside the template