depr(App.rs): deprecates old App methods in prep for v3

A full list of deprecations is:

* App::version_message -> App::mut_arg
* App::version_short -> App::mut_arg
* App::help_message -> App::mut_arg
* App::help_short -> App::mut_arg
* App::from_yaml -> serde
* App::usage -> App::override_usage (+no longer assumes leading \t)
* App::help -> App::override_help
* App::template -> App::help_template
* App::args_from_usage -> App::args(&str)
* App::arg_from_usage -> App::arg(Arg::from)
* App::write_help -> &self -> &mut self (#808)
* App::gen_completions -> clap_completions::generate
* App::gen_completions_to -> clap_completions::generate_to
* App::get_matches_safe -> App::try_get_matches (#950 Lib Blitz naming
consistency)
* App::get_matches_from_safe -> App::try_get_matches_from (#950 Lib Blitz
naming consistency)
* App::get_matches_safe_borrow -> App::try_get_matches_from_mut (#950 Lib
Blitz naming consistency)