feat: add option for generic lazy-loading placeholder color

[abaicus]

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Changes proposed in this Pull Request:

• Adds setting for generic lazyload placeholder color;

Screenshots

Closes #624.

How to test the changes in this Pull Request:

1. Turn on Enable generic lazyload placeholder under Settings > Lazyload;
2. The button for the placeholder color should appear under the setting, allowing you to select a color;
3. This color will be empty by default - the placeholder being transparent;
4. Create a page containing images - you can also lower the Exclude the first X images from lazyload to 0 so all images on the page get lazyloaded (this is not going to work with DAM images as they are not lazyloaded yet);
5. You can enable network throttling under the browser inspector settings (Slow 3G) should be enough, so you have time to see the placeholders;
6. The placeholder for the lazyloaded images should have the color you select in the color picker;

How the placeholders should look:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[irinelenache]

@abaicus Nice work 🚀 Tested and found a single thing, don't know if it's an issue or it's expected:

• If i'm using a slower network (slow 3G), refresh the page and wait for the entire page to load without scrolling, the placeholders will turn from blurry to straight color and then to the actual image. Here is a video with the behaviour https://vertis.d.pr/v/PQeeYF

[abaicus]

@irinelenache yes, this would be expected. That's how it used to work with generic placeholder before as well.

[irinelenache]

@abaicus Thanks for letting me know, i'll move this to Ready to merge as there are no other issues ✔️

[pirate-bot]

🎉 This PR is included in version 3.10.0 🎉

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