Add support for actix-web 4

[SorenHolstHansen]

As the title says. It currently breaks on an actix web route with

the trait bound `<index as actix_web::service::HttpServiceFactory>::register::index::{closure#0}::maud::PreEscaped<std::string::String>: actix_web::Responder` is not satisfied
the following other types implement trait `actix_web::Responder`:
  &'static [u8]
  &'static str
  &std::string::String
  (R, actix_web::http::StatusCode)
  actix_http::responses::builder::ResponseBuilder
  actix_web::CustomizeResponder<T>
  actix_web::Either<L, R>
  actix_web::HttpResponse<B>
and 13 others
required because of the requirements on the impl of `actix_web::Responder` for `std::result::Result<<index as actix_web::service::HttpServiceFactory>::register::index::{closure#0}::maud::PreEscaped<std::string::String>, actix_web::Error>

[SanchithHegde]

Related: #330, #340.

TL;DR: Use maud as a git dependency.

[SorenHolstHansen]

Thanks