Fixes editing of entries with '%' and '"' characters in title or URL.

Adds new function `yourls_esc_inputfield()` in file `includes/functions-html.php`:
Filter  `"` (double quotation mark) into `&quot;` for glueing strings into HTML `<input …>` form fields.
Percent signs `%` are converted to HTML entity `&#37;` to prevent `sprintf()` choking from too many `%`.
Please note: The function expects urldecoded input, so `$url` has to be piped through `rawurldecode()` first. 


Removed doubling of percent signs as it should not be required anymore.