Web: Fix 'Download' not triggering until second attempt

[infinite-persistence]

Issue

Closes #4669: Download doesn't trigger on web until 2nd attempt

The issue only happens when Autoplay is disabled in the User Settings and the video hasn't been loaded when Download is clicked.

The following code:
if (didClickDownloadButton && streamingUrl)
didn't triggered because:

1. streamingUrl has not resolved yet when the Effect ran.
2. When it did resolve, the parent component was also notified and unmounted things, causing didClickDownloadButton to reset.

Approach

• Change the state to the persisted version to deal with the unmounting.
• As a secondary precaution against any unexpected mismatch from the mounting/unmounting, the state was changed from boolean to string, with the compared value being the current uri when the Download button was clicked.

[neb-b]

I think a potentially better fix would be to make sure this doesn't get removed from the page when while streamingUrl is being fetched. Then we don't need a new persisted value for every download you click.

If that's not possible, we should create a new effect useSessionState which is the same as usePersistedState except it uses sessionStorage instead of localStorage so the value dies when you leave the page.

[infinite-persistence]

Rework
Avoid the unnecessary unmounting by not using a conditional section wrapper within a return statement. React probably couldn't do the diffs when the conditional is at a section level.