Fill image and link prompts to 'https://' by default

[roipoussiere]

When promptURLs is set to true, fill image and link prompts to 'https://' by default, like this:

• it's easier to fill an url if we know it, because we don't have to type 'https://';
• it helps users to understand that something starting with https:// should be usually filled in the pop-up;
• if the user click Ok without filling nothing, this inserts [](https://) / ![](https://), so the default behavior is consistent with promptURLs=false, which inserts these values;
• this doesn't bother the user, because he can copy-paste url like before;