MzpNavigation doesn't check for MzpSupports availability correctly

## Description

`MzpNavigation.menuButtonVisible()` and `MzpNavigation.setAria()` don't check for `window.MzpSupports` availability the way it was intended to.

### Steps to reproduce

https://github.com/mozilla/protocol/blob/2820247231efd9cb0a2437375506416e84f1b082/assets/js/protocol/navigation.js#L209-L210

### Expected result

```js
typeof MzpSupports !== 'undefined'
```

### Actual result

```js
MzpSupports !== 'undefined'
```

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MzpNavigation doesn't check for MzpSupports availability correctly #1085

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	MzpNavigation.setAria = () => {
	if (MzpSupports !== 'undefined' && MzpSupports.intersectionObserver) {

MzpNavigation doesn't check for MzpSupports availability correctly #1085

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