[ticket/12730] Add Google+ profile field

I had to create a new custom profile field type since Google+ formats URLs differently for integer IDs and custom URLs. For custom URLs it adds a + before the URL which makes it hard to validate or format, created a new offshoot of type_string in order to compensate for that.
or try to not use a new template file.
Just overwrite the method `get_template_filename()` of the base class in your instance:
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/profilefields/type/type_base.php#L69

Just add something like:
```
	/**
* {@inheritDoc}
*/
public function get_template_filename()
{
return 'profilefields/string.html';
}
```