[FIX] web_editor: prevent opening linktool on uneditable links

Before this commit
When trying to open the linktool on link that are not
`isContentEditable`, the linktool was initialized and not
showed in the sidebar.

After this commit
As it does not make sense to edit a link that is not editable,
prevent the linktool from being initialized in the first place.

task-2802592

closes odoo/odoo#87137

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>