In the installer, allow site template screenshots to be provided as data URIs
>>> [!note] Migrated issue
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Reported by: [phenaproxima](https://www.drupal.org/user/205645)
Related to !813
>>>
<h3 id="summary-problem-motivation">Problem/Motivation</h3>
<p>At the moment, the installer expects site templates to have a screenshot, called screenshot.webp, in the recipe directory. This gets read and passed to the theme as an inline data URI.</p>
<p>In light of recent changes to Recipe Installer Kit to support injecting arbitrary lists of additional site templates, we need to also support the screenshot being passed to the theme <em>as a data URI</em>, rather than always assuming we have a file we can read.</p>
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