After OAUTH, SAML auth completion, Redirect to App custom url scheme with token data as query params. (#16447)

* Added redirection after Auth complete

* Fixed gofmt

* Handling error while parsing the url, added util function to check for a valid mobile redirect url

* Added check for custom scheme url validation

* Added test to verify custom schele url

* Added mobile message screens

* Translation strings for mobile screens

* Added mobile specific screens for success and error

* Added error logs and changed variable name for consistency with oauth.go

* i18n fix

* Reusing assigned variable instead of map

* Added AppCustomUrlScheme property

* Code refactor and removed dependency from cookies to build the final url

* Changed util function

* Updated util test

* Fixed go lint

* Code refactor and unsused code removed

* Code refactor

* simplified boolean checks

* Added support of whitelist of appCustomURLSchemes

* Changed i18 en

* Fixed validating redirecturl for web

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

utils/utils.go

@@ -11,6 +11,8 @@ import (
 	"strings"
 
 	"github.com/pkg/errors"
+
+	"github.com/mattermost/mattermost-server/v5/model"
 )
 
 func StringInSlice(a string, slice []string) bool {
@@ -173,3 +175,43 @@ func GetUrlWithCache(url string, cache *RequestCache, skip bool) ([]byte, error)
 	cache.Date = resp.Header.Get("Date")
 	return cache.Data, err
 }
+
+// Append tokens to passed baseUrl as query params
+func AppendQueryParamsToURL(baseUrl string, params map[string]string) string {
+	u, err := url.Parse(baseUrl)
+	if err != nil {
+		return ""
+	}
+	q, err := url.ParseQuery(u.RawQuery)
+	if err != nil {
+		return ""
+	}
+	for key, value := range params {
+		q.Add(key, value)
+	}
+	u.RawQuery = q.Encode()
+	return u.String()
+}
+
+// Validates RedirectURL passed during OAuth or SAML
+func IsValidWebAuthRedirectURL(config *model.Config, redirectURL string) bool {
+	u, err := url.Parse(redirectURL)
+	if err == nil && (u.Scheme == "http" || u.Scheme == "https") {
+		if config.ServiceSettings.SiteURL != nil {
+			siteUrl := *config.ServiceSettings.SiteURL
+			return strings.Index(strings.ToLower(redirectURL), strings.ToLower(siteUrl)) == 0
+		}
+		return false
+	}
+	return true
+}
+
+// Validates Mobile Custom URL Scheme passed during OAuth or SAML
+func IsValidMobileAuthRedirectURL(config *model.Config, redirectURL string) bool {
+	for _, URLScheme := range config.NativeAppSettings.AppCustomURLSchemes {
+		if strings.Index(strings.ToLower(redirectURL), strings.ToLower(URLScheme)) == 0 {
+			return true
+		}
+	}
+	return false
+}