Move Low Bandwidth feature to devtools.

[Half-Shot]

The "low bandwidth" mode has sat in draft for a very long time and needs to find a new home. Having chatted with the original developer, we decided it would be best to:

• Rename this. This is not the new low bandwidth mode, but the original effort to just reduce the footprint.
• The description is completely wrong. This updates it to describe exactly what it does.
• The feature should live in devtools. We don't expect anyone to be turning this on outside of a demo environment. This is NOT a supported production environment but it is a useful tool to show a reduced-featureset Matrix client.

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)