Mweb 853 querystring

@rogatty @nandy-andy @RafLeszczynski This hides attribution bar on hideAttr=1 appended to querystring.
I remember we were talking about setting `hideAttr` to an integer: `1`. Now, I'm thinking about just checking if it was set:
```js
mapData.hideAttr = typeof(req.query.hideAttr) !== 'undefined';
```
then on the client side they can just pass:
* `?hideAttr`,
* `?hideAttr=1`,
* `?hideAttr=true`,
* or even `?hideAttr=yesPlease`

It's few bytes more and it's implicit instead of explicit, though. @RafLeszczynski, @aquilax what do you think?