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>

routers/web/repo/milestone.go

@@ -238,7 +238,7 @@ func DeleteMilestone(ctx *context.Context) {
 // MilestoneIssuesAndPulls lists all the issues and pull requests of the milestone
 func MilestoneIssuesAndPulls(ctx *context.Context) {
 	milestoneID := ctx.PathParamInt64("id")
-	projectID := ctx.FormInt64("project")
+	projectIDs := parseProjectIDsFromQuery(ctx)
 	milestone, err := issues_model.GetMilestoneByRepoID(ctx, ctx.Repo.Repository.ID, milestoneID)
 	if err != nil {
 		if issues_model.IsErrMilestoneNotExist(err) {
@@ -260,7 +260,7 @@ func MilestoneIssuesAndPulls(ctx *context.Context) {
 	ctx.Data["Title"] = milestone.Name
 	ctx.Data["Milestone"] = milestone
 
-	prepareIssueFilterAndList(ctx, milestoneID, projectID, optional.None[bool]())
+	prepareIssueFilterAndList(ctx, milestoneID, projectIDs, optional.None[bool]())
 
 	ret := issue.ParseTemplatesFromDefaultBranch(ctx.Repo.Repository, ctx.Repo.GitRepo)
 	ctx.Data["NewIssueChooseTemplate"] = len(ret.IssueTemplates) > 0