Add UIFeature.allowChatExport

[terminalix-png]

Changes

Added the ability to hide the chat export button via UIFeature in config.json

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible). The new code is very minor
• 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)

[CLAassistant]

All committers have signed the CLA.

[mxandreas]

The ability to hide the export is generally appreciated, as there are use cases where users are concerned about data exfiltration and that being made very easy for the end users. However, we should double check if there is an option to hide it already using a server-side setting.