[Bug] Hypothes.is not updated after opening new file.

There is an upload button for hypothesis's pdf.js view:
However, when i upload a local file via this button, the hypothes.is sidebar remain the content for original file.

I suppose with this bug fixed, it's possible to use hypothes.is on android device without hosting file on server, as i can use a [dummy page](https://via.hypothes.is/https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf) and upload the file, i am actually considering developing a PWA out of it, making the mobile use of hypothes.is easier.

Now, after playing around with the code, it seems the problem can be fixed by modifying `webViewerFileInputChange` in `web/viewer.js` by adding the following at the end of the function

```javascript
function loadHypothesis() {
PDFViewerApplication.initializedPromise.then(() => {
const embedScript = document.createElement('script');
embedScript.src = 'https://hypothes.is/embed.js';
document.body.appendChild(embedScript);
});
}

function unloadHypothesis() {
var appLinkEl =
document.querySelector('link[type="application/annotator+html"]');
appLinkEl.dispatchEvent(new Event('destroy'));
};

function reloadHypothesis(){
unloadHypothesis(); loadHypothesis();
}

reloadHypothesis()
```

I don't know it's the correct way, but Do please consider fix this bug!