Steps to replicate the issue (include links if applicable):

• Create an empty page with the content model of 'javascript'
• Now edit the page to include a string with content along the lines of "{{subst:template}}"
• Go to 'show preview' or 'show changes'

What happens?: The template is substituted

What should have happened instead?: The template should not be substituted

Software version (skip for WMF-hosted wikis like Wikipedia): 1.39.0

Other information (browser name/version, screenshots, etc.): Safari