Lazy loading of table and iframe panels

[egonw]

Fixes #2602

Description

The patch uses the Intersection Observer API. For each panel, it creates a unique observer. When a panel becomes visible, the (unique) handler is called. This is followed by an unobserve to ensure that the SPARQL query is run only once.

Caveats

This browser compability page shows that the Intersection Observer API is widely available, but not on old browsers: https://caniuse.com/intersectionobserver

• Breaking change (unsupported for old browsers)
• This change requires a documentation update
  • I have made corresponding changes to the documentation
• This change requires new dependencies (please list)

Testing

Open up as many pages as possible and see if you see irregularities.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have not used code from external sources without attribution
• I have considered accessibility in my implementation
• There are no remaining debug statements (print, console.log, ...)

[Daniel-Mietchen]

This seems to work fine, and the console message comments can probably be removed.

However, I opened a separate ticket for a related issue:

• Ensure the Reload button is there even if we get a rate limit error #2613

[egonw]

@fnielsen, can you please let me know if this patch is good to go?