Add more linters to improve code readability (#19989)

Add nakedret, unconvert, wastedassign, stylecheck and nolintlint linters to improve code readability

- nakedret - https://github.com/alexkohler/nakedret - nakedret is a Go static analysis tool to find naked returns in functions greater than a specified function length.
- unconvert - https://github.com/mdempsky/unconvert - Remove unnecessary type conversions
- wastedassign - https://github.com/sanposhiho/wastedassign -  wastedassign finds wasted assignment statements.
- notlintlint -  Reports ill-formed or insufficient nolint directives
- stylecheck - https://staticcheck.io/docs/checks/#ST - keep style consistent
  - excluded: [ST1003 - Poorly chosen identifier](https://staticcheck.io/docs/checks/#ST1003) and [ST1005 - Incorrectly formatted error string](https://staticcheck.io/docs/checks/#ST1005)

modules/activitypub/user_settings.go

@@ -35,11 +35,11 @@ func GetKeyPair(user *user_model.User) (pub, priv string, err error) {
 // GetPublicKey function returns a user's public key
 func GetPublicKey(user *user_model.User) (pub string, err error) {
 	pub, _, err = GetKeyPair(user)
-	return
+	return pub, err
 }
 
 // GetPrivateKey function returns a user's private key
 func GetPrivateKey(user *user_model.User) (priv string, err error) {
 	_, priv, err = GetKeyPair(user)
-	return
+	return priv, err
 }