Support for Custom URI Schemes in OAuth2 Redirect URIs (#37356)

Fix #34349

By the way, remove `(ctx *APIContext) HasAPIError() ` and `(ctx
*APIContext) GetErrMsg()` because they do nothing, the error handling
has been done in API's middeware

The existing OAuth2 tests were not quite right, refactored them together

modules/setting/oauth2.go

@@ -99,6 +99,7 @@ var OAuth2 = struct {
 	JWTClaimIssuer             string `ini:"JWT_CLAIM_ISSUER"`
 	MaxTokenLength             int
 	DefaultApplications        []string
+	CustomSchemes              []string
 }{
 	Enabled:                    true,
 	AccessTokenExpirationTime:  3600,