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/templates/util_slice.go

@@ -6,6 +6,11 @@ package templates
 import (
 	"fmt"
 	"reflect"
+	"slices"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/util"
 )
 
 type SliceUtils struct{}
@@ -33,3 +38,29 @@ func (su *SliceUtils) Contains(s, v any) bool {
 	}
 	return false
 }
+
+// JoinInt64 joins a slice of int64 values into a comma-separated string.
+func (su *SliceUtils) JoinInt64(values []int64) string {
+	if len(values) == 0 {
+		return ""
+	}
+	strs := make([]string, len(values))
+	for i, v := range values {
+		strs[i] = strconv.FormatInt(v, 10)
+	}
+	return strings.Join(strs, ",")
+}
+
+func (su *SliceUtils) JoinToggleIDs(values []int64, target int64) (ret struct {
+	IsIncluded bool
+	ToggledIDs string
+},
+) {
+	ret.IsIncluded = slices.Contains(values, target)
+	if ret.IsIncluded {
+		ret.ToggledIDs = su.JoinInt64(util.SliceRemoveAll(slices.Clone(values), target))
+	} else {
+		ret.ToggledIDs = su.JoinInt64(append(values, target))
+	}
+	return ret
+}