Pasting text into signature preview is possible

[armhub]

If you select the "None" or "Default" email signature it is possible to paste text into the preview field. This should not be possible. Seen in the desktop client.

Test notes

• [x] Set your signature to Default or None.
• [x] Click on the preview and try to paste some text into it.
  • [x] Verify no text is inserted.
• [x] Save the signature and try to edit it again.
  • [ ] Verify the signature preview is read only.

[ghost]

Tried to fix but pasting happens before event gets triggered

[paw-hub]

Apparently you can actually edit the signature preview normally too.

[nif-tutao]

This issue fails testing as the signature preview box is editable and not read-only. If you select something in the preview box, you can actually delete it.

[charlag]

We disabled manual editing but squire still does what it does so we need to disable squire somehow. Destroying it will not work as we pass all values though it.

[charlag]

Editor or HtmlEditor would need a complete rewrite to make this bullet-proof, I will take this out of the milestone