Fix editing external links

[mlocati]

• Call canEditPageProperties instead of canDeletePage to check if a user can edit an existing external link (ref)
• Add a missing </div>
• Submit button text should be Save instead of Add when editing an existing external link
• Avoid deprecated code
• Avoid accessing undefined vars (ref. Get rid of every warning & code cleanup #4723)

[mlocati]

I also normalized the size of the Add External Link and Edit External Link dialogs.
Before we had the height of the Add dialog was 260 pixels (a bit too smal), and the height of the Edit dialog was 400 pixels (ways too big).