Add loading yaml label template files (#22976) (#23232)

Backport #22976

Extract from #11669 and enhancement to #22585 to support exclusive
scoped labels in label templates

* Move label template functionality to label module
* Fix handling of color codes
* Add Advanced label template

Co-authored-by: Lauris BH <lauris@nix.lv>

modules/label/label.go

@@ -0,0 +1,46 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package label
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+// colorPattern is a regexp which can validate label color
+var colorPattern = regexp.MustCompile("^#?(?:[0-9a-fA-F]{6}|[0-9a-fA-F]{3})$")
+
+// Label represents label information loaded from template
+type Label struct {
+	Name        string `yaml:"name"`
+	Color       string `yaml:"color"`
+	Description string `yaml:"description,omitempty"`
+	Exclusive   bool   `yaml:"exclusive,omitempty"`
+}
+
+// NormalizeColor normalizes a color string to a 6-character hex code
+func NormalizeColor(color string) (string, error) {
+	// normalize case
+	color = strings.TrimSpace(strings.ToLower(color))
+
+	// add leading hash
+	if len(color) == 6 || len(color) == 3 {
+		color = "#" + color
+	}
+
+	if !colorPattern.MatchString(color) {
+		return "", fmt.Errorf("bad color code: %s", color)
+	}
+
+	// convert 3-character shorthand into 6-character version
+	if len(color) == 4 {
+		r := color[1]
+		g := color[2]
+		b := color[3]
+		color = fmt.Sprintf("#%c%c%c%c%c%c", r, r, g, g, b, b)
+	}
+
+	return color, nil
+}