Bookmarklets to beat servers that trip up on `?u=http://`

Trick : instead of sending to `index.php?u=http://blah/hey`, we split the URL into protocol, slashes and the rest (`http:`, `//` and `blah/hey`)

See #1512 and [my comment](https://github.com/YOURLS/YOURLS/issues/1512#issuecomment-26415751)

- new function: `yourls_get_protocol_slashes_and_rest( $url )`
- fix "Prefix n' Shorten" bookmarklet
- fix 4 regular bookmarklets
- while I'm at it, add another "social" bookmarklet to share on Tumblr