Fix issue on terms page with mermaid plots

What does this MR do and why?

Fix issue on terms page with mermaid plots

When the terms page contains a mermaid plot it is rendered in a sandboxed iframe (/-/sandbox/mermaid), however the URL is redirected again to the terms page. Here we resolve this issue.

Changelog: changed

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Before	After

How to set up and validate locally

1. Enforce terms of service in the admin interface: /admin/application_settings/general#js-terms-settings
2. Add a mermaid plot, for example:

```mermaid
graph TD;
  A-->B;
  A-->C;
  B-->D;
  C-->D;
```

1. Check the terms page: /-/users/terms