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>

services/context/base_form.go

@@ -7,6 +7,7 @@ import (
 	"strconv"
 	"strings"
 
+	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/optional"
 	"code.gitea.io/gitea/modules/util"
 )
@@ -35,6 +36,11 @@ func (b *Base) FormStrings(key string) []string {
 	return nil
 }
 
+func (b *Base) FormStringInt64s(key string) []int64 {
+	vals, _ := base.StringsToInt64s(strings.Split(b.FormString(key), ","))
+	return vals
+}
+
 // FormTrim returns the first value for the provided key in the form as a space trimmed string
 func (b *Base) FormTrim(key string) string {
 	return strings.TrimSpace(b.Req.FormValue(key))