Refactor context flash msg and global variables (#33375)

1. add `GetSiteCookieFlashMessage` to help to parse flash message
2. clarify `handleRepoHomeFeed` logic
3. remove unnecessary global variables, use `sync.OnceValue` instead
4. add some tests for `IsUsableUsername` and `IsUsableRepoName`

modules/validation/glob_pattern_test.go

@@ -19,40 +19,40 @@ func getGlobPatternErrorString(pattern string) string {
 	return ""
 }
 
-var globValidationTestCases = []validationTestCase{
-	{
-		description: "Empty glob pattern",
-		data: TestForm{
-			GlobPattern: "",
-		},
-		expectedErrors: binding.Errors{},
-	},
-	{
-		description: "Valid glob",
-		data: TestForm{
-			GlobPattern: "{master,release*}",
-		},
-		expectedErrors: binding.Errors{},
-	},
-
-	{
-		description: "Invalid glob",
-		data: TestForm{
-			GlobPattern: "[a-",
-		},
-		expectedErrors: binding.Errors{
-			binding.Error{
-				FieldNames:     []string{"GlobPattern"},
-				Classification: ErrGlobPattern,
-				Message:        getGlobPatternErrorString("[a-"),
-			},
-		},
-	},
-}
-
 func Test_GlobPatternValidation(t *testing.T) {
 	AddBindingRules()
 
+	globValidationTestCases := []validationTestCase{
+		{
+			description: "Empty glob pattern",
+			data: TestForm{
+				GlobPattern: "",
+			},
+			expectedErrors: binding.Errors{},
+		},
+		{
+			description: "Valid glob",
+			data: TestForm{
+				GlobPattern: "{master,release*}",
+			},
+			expectedErrors: binding.Errors{},
+		},
+
+		{
+			description: "Invalid glob",
+			data: TestForm{
+				GlobPattern: "[a-",
+			},
+			expectedErrors: binding.Errors{
+				binding.Error{
+					FieldNames:     []string{"GlobPattern"},
+					Classification: ErrGlobPattern,
+					Message:        getGlobPatternErrorString("[a-"),
+				},
+			},
+		},
+	}
+
 	for _, testCase := range globValidationTestCases {
 		t.Run(testCase.description, func(t *testing.T) {
 			performValidationTest(t, testCase)