Track Downloads through Universal Viewer

[tpendragon]

The download pane in Universal Viewer:

1. Doesn't fire any javascript events (so we can't bind to an event and send a custom event)
2. Doesn't count as an outbound link (https://plausible.io/docs/outbound-link-click-tracking doesn't work)
3. Doesn't count as a "File Download" (https://plausible.io/docs/file-downloads-tracking doesn't work)
4. DOES use window.open to open a link. (https://github.com/UniversalViewer/universalviewer/blob/main/src/content-handlers/iiif/extensions/uv-openseadragon-extension/DownloadDialogue.tsx#L431)

Thus we can fire a custom event by, if Universal Viewer is loaded, overriding window.open in the viewer. This also adds an "?embed_host" property that can be added to the iframe, and sends the environment it's sending as. This is done via the documentation at https://plausible.io/docs/custom-props/for-pageviews#2-add-your-custom-properties-to-your-plausible-snippet.

Finally, this uses manual for Plausible, which means it doesn't automatically track page views - I don't think we care how many people go to /viewer, just how many people interact with that viewer.

The documentation for sending a custom event with properties in Plausible is here: https://plausible.io/docs/custom-props/for-custom-events#2-using-the-manual-method

Closes #6376 Closes #6377

This will miss any downloads done from within Figgy. We could do that, but I'm not sure it's necessary - that's not a public interface our patrons will be accessing.

[hackartisan]

Are there any plausible docs you used to work this out? It would be nice to have the links to them in the description of the PR.

[tpendragon]

@hackartisan Good idea! Added some links.

[tpendragon]

Drafted so I can add some tests of this plausible functionality.