Update preview when selecting a new template

Summary

Changing a template (issues or merge requests) while on "Preview" tab does not refresh the content.

Steps to reproduce

• Go to Issues > New Issue
• Choose a template
• Select "Preview" tab
• Choose another template

Expected behavior

Update the new template directly in Preview mode.
As mentioned in #21655 (closed) there should be a warning if text will be lost by switching templates.

Actual behaviour

The old template is still shown. You have to switch to "Write" tab and then again to "Preview" to see the current preview.