Use element rights for delete buttons

Closes #790 (closed)

This allows an element's creator to delete in three out of four situations:

• The Delete the current element action
• The Delete button of the element edition shown via the ✏ icon from the children tree
• The 🗑 icon next to the element's name in the details panel

This does not cause the 🗑 icon to show on element thumbnails from lists, as it would for project admins, because rights is not available on element lists and making 20 to 500 API queries per page is not really a good idea.