move issue filters to shared template (#25729)

Issue filters are being used on repo list page and on milestone issues
page, and the code is mostly duplicated.

This PR does the following changes:
- move issue filters into a shared template
- allow filtering milestone issues by project, so no need to hide this
filter on milestone issues page
- remove some dead code (e. g. issue actions in milestone issues
template)
- fix label filter dropdown width

---------

Co-authored-by: 6543 <6543@obermui.de>

routers/web/repo/milestone.go

@@ -264,6 +264,7 @@ func DeleteMilestone(ctx *context.Context) {
 // MilestoneIssuesAndPulls lists all the issues and pull requests of the milestone
 func MilestoneIssuesAndPulls(ctx *context.Context) {
 	milestoneID := ctx.ParamsInt64(":id")
+	projectID := ctx.FormInt64("project")
 	milestone, err := issues_model.GetMilestoneByRepoID(ctx, ctx.Repo.Repository.ID, milestoneID)
 	if err != nil {
 		if issues_model.IsErrMilestoneNotExist(err) {
@@ -289,7 +290,7 @@ func MilestoneIssuesAndPulls(ctx *context.Context) {
 	ctx.Data["Title"] = milestone.Name
 	ctx.Data["Milestone"] = milestone
 
-	issues(ctx, milestoneID, 0, util.OptionalBoolNone)
+	issues(ctx, milestoneID, projectID, util.OptionalBoolNone)
 
 	ret, _ := issue.GetTemplatesFromDefaultBranch(ctx.Repo.Repository, ctx.Repo.GitRepo)
 	ctx.Data["NewIssueChooseTemplate"] = len(ret) > 0