Fix styling issues related to sandboxed mermaid charts

What does this MR do and why?

It sets the iframe element's width during creation. Setting it dynamically after the DOM is created, doesn't affect it all, and causes styling issues with some specific mermaid charts.

The issues can be reproduced at https://staging.gitlab.com/djadmin/test-mermaid-charts/-/issues/3

Screenshots or screen recordings

before	after

Checked for any regression

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:sandboxed_mermaid)

2. Create an issue / MR and generate mermaid diagrams

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.