fix: build site with CSP disabled

[welpo]

Description

The site failed to build with csp disabled. The reason was the CSP block was setting the page_image variable, but the last styling section of head.html had an if checking for page.extra.image but using page_image if set.

Notes:

• Might want to set page_image outside the csp block;
• In any case, I believe the if should check for the variable that will later be used (whether it's page.extra.image or page_image).

Motivation

Site should build regardless of csp setting.

Logs (if applicable)

Checklist:

<!— Go over all the following points and put an x in all the boxes that apply. —>

• My code follows the code style of this project.
• I have updated the documentation accordingly.

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[charlesrocket]

Looks like I missed this one, thanks!