Validate user URL input to ensure protocol is present

closes #1623
- added protocol validation to isURL on user settings validator.
- added placeholder URL to user profile template
- ref: https://github.com/sethlilly/Vapor/issues/10
- ref: https://github.com/oswaldoacauan/ghostium/issues/38
Merge pull request #2734 from shindakun/userurl

Validate user URL input to ensure protocol is present