Add custom HTML blocks to admin settings

[icewindow]

Overview

The possibility to write custom CSS is already present in the Appearance settings. A way to add custom HTML to the page would compliment this nicely.

Motivation

Admins may wish to add custom Script snippets to the website. This could be custom tracking code (see #818) or any number of custom JavaScript applications. Currently there is no mechanism to do so, except for maybe hijacking templates via Docker mounts or similar mechanisms.

Possible implementation details

There should be two text fields to allow adding custom HTML into the head, possibly before the closing tag, and another one to add custom HTML to the end of the body. Two fields because some scripts are required to be placed in head, some are required to be placed in body.

Considerations

This opens up the possibility of XSS and similar attacks. There needs to be a warning to not copy-paste code which is not understood what it does into these text fields.
A different precaution could be to make the fields lockable via environment variables.