Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
> What are your thoughts about this: We still perform the protocol check. But if the protocol is unrecognized, instead of preventing the addition of the link, we notify the user that the URI wasn't recognized by zotero and ask them to double-check their input.

No, I don't think there's any need for that. If they enter it incorrectly, they can recreate it. This isn't a comprehensive list, so there's no point pretending it is, as long as we don't have security concerns.

Same with pre-pending http:// — if it's not what they want, they can just recreate and enter how they want. No point throwing up unnecessary roadblocks.

> One idea I have is to enable a checkbox to the left of the address box with a value "http://" and by default the box is checked.

I don't see a need for this. Adjusting the user's input in this case really isn't a big deal.