Allow multiple projects per issue and pull requests (#36784)

Add ability to add and remove multiple projects per issue
and pull request.

Resolve #12974

---------

Signed-off-by: Icy Avocado <avocado@ovacoda.com>
Co-authored-by: Tyrone Yeh <siryeh@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: OpenCode (gpt-5.2-codex) <opencode@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

modules/util/util.go

@@ -15,6 +15,8 @@ import (
 	"strings"
 	"sync"
 
+	"code.gitea.io/gitea/modules/container"
+
 	"golang.org/x/text/cases"
 	"golang.org/x/text/language"
 )
@@ -291,3 +293,21 @@ func NormalizeStringEOL(input string) string {
 	// Other than this, we should respect the original content, even leading or trailing spaces.
 	return UnsafeBytesToString(NormalizeEOL(UnsafeStringToBytes(input)))
 }
+
+func DiffSlice[T comparable](oldSlice, newSlice []T) (added, removed []T) {
+	oldSet := container.SetOf(oldSlice...)
+	newSet := container.SetOf(newSlice...)
+
+	addedSet, removedSet := container.Set[T]{}, container.Set[T]{}
+	for _, v := range newSlice {
+		if !oldSet.Contains(v) && addedSet.Add(v) {
+			added = append(added, v)
+		}
+	}
+	for _, v := range oldSlice {
+		if !newSet.Contains(v) && removedSet.Add(v) {
+			removed = append(removed, v)
+		}
+	}
+	return added, removed
+}