fetch imagery directly from ELI for instant updates

[k-yle]

blocked by either osmlab/editor-layer-index#2927 or osmlab/editor-layer-index#2530, depending on what option the ELI maintainers prefer.

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This PR makes iD use the latest version of editor-layer-index, no need to wait for the next release anymore.

• Closes Fetch imagery weekly #9730
• Closes Regular releases with updated ELI #8466
• makes Deploy an iD preview site as a PR workflow osmlab/editor-layer-index#2625 much easier to implement
• fixe­s Heads up: github security scanning now detects mapbox api keys osmlab/editor-layer-index#2904 from iD's side (since we don't commit imagery.json into our git repo anymore)
• removes the need for Show last update date of ELI below Layer list #10238

[k-yle]

if osmlab/editor-layer-index#2927 is accepted, then this line needs to change to:

Suggested change

	|| 'https://cdn.jsdelivr.net/gh/osmlab/editor-layer-index@gh-pages/';
	|| 'https://cdn.jsdelivr.net/npm/@openstreetmap/editor-layer-index@{version}/';

or alternatively, if osmlab/editor-layer-index#2530 is accepted, then this needs to change to:

Suggested change

	|| 'https://cdn.jsdelivr.net/gh/osmlab/editor-layer-index@gh-pages/';
	|| 'https://cdn.jsdelivr.net/gh/osmlab/editor-layer-index@v1/';

currently it's using gh-pages just to make the PR demo work, it can't be merged as-is.