Remove scheme validation from URLValidator or provide all IANA acceptable schemes as defaults

[Marcin Nowak]

The problem

There are defined only four schemes as default: http,https,ftp,ftps.
But URL can have many many more valid schemes.
HTTP and FTP as good defaults are invalid - they are too limited.

There is no simple way to extend these schemes nor disable scheme validation.
We must extend URLField and redeclare default_validators class property, which is too complex for that simple case.

Possible solutions

• disable scheme validation and enable it only when user defines allowed_schemes directly in URLField (backward incompatible)
• provide allowed_schemes as an optional argument for URLField init()
• provide all valid schemes as default set (based on ​https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml)

Preffered solution

Probide all (IANA) valid schemes as default together with possibility to change them by URLField init().