Link preview: toggle should be hidden if link preview is not possible

[mejo-]

Describe the bug
When previewPossible is false, the toggle to change to preview should be hidden in my opinion.

At the moment, you can still toggle between text and preview in this case, but it has no user-visible effect.

To Reproduce
Steps to reproduce the behavior:

1. Add a link that only contains a hash (e.g. #section)
2. See the text/preview toggle left to this link
3. Change to 'Display as preview'
4. See nothing changing

Expected behavior
The toggle should be hidden in these cases.

Server details:

• Nextcloud version: 29