Draft: Add custom css field to admin appearance settings

What does this MR do and why?

Quick n dirty attempt at Feature request - add an easily modifiable "cus... (#15199)

Allow uploading custom CSS file to serve for an instance

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit Admin section, go to Appearance
2. Add some custom CSS to the textarea, click Save

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.