Escape special characters to avoid syntax errors

[vcanales]

Special characters were causing patterns to crash the site due to syntax errors. In this change, we make sure that these characters are escaped before being printed to the pattern.

Tested using the string "ABC/'"-.`';//\''``" as alt text for a template image.

Fixes: #349

[mikachan]

I tested this using TT3. I inserted an image on the home template using, "ABC/'"-.`';//''``", as the alt text. The theme was exported and activated successfully with no error messages, and I can see the image with the correct alt text.

LGTM 👍