Download visualisations as images with attribution text overlay

[carlhiggs]

Popup charts and other visualisations should be downloadable, ideally with a text overlay indicating attribution.

The overlay text might be brief and general (e.g. "https://transporthealthimpacts.org (2024)"), or perhaps more specific (e.g. the URL that could direct users to the interactive version of that specific clicked feature to which the image relates).

I have implemented the button using html2canvas (1ee789f), however the text overlay has proved complicated/unpredictable to position. More work is required.

[carlhiggs]

It wasn't straightforward to implement the attribution overlay --- there are complexities that can make spacing/positions difficult to predict. But downloading content has been implemnted. I may close this issue soon, as the downloading was the main thing ---attribution overlay is a 'nice to have'.... There are other priorities that I'll likely get to first.