Add User Apps (#266) #1192

Job	Run time
Build docs	1m 54s
Rustfmt	16s
Clippy	1m 0s
Test (MSRV)	1m 22s
Test (stable)	2m 6s
Test (beta)	2m 7s
Test (nightly)	1m 52s
Test (macOS)	1m 26s
Test (Windows)	5m 7s
Test (no features)	1m 18s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 19s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 31s
	21m 18s

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