feature to be able to filter project boards by milestones (#36321)

This pull request adds milestone filtering support to both repository
and organization project boards. Users can now filter project issues by
milestone, similar to how they filter by label or assignee. The
implementation includes backend changes to fetch and filter milestones,
as well as frontend updates to display a milestone filter dropdown in
the project board UI.

**Milestone filtering support:**

* Added support for filtering project board issues by milestone in both
repository and organization contexts, including handling for "no
milestone" and "all milestones" options. (`routers/web/repo/projects.go`
[[1]](diffhunk://#diff-5cba331a1ddf1eea017178cfefaaff9ad72a4b05797fb84bf508b0939aae2972R316-R330)
[[2]](diffhunk://#diff-5cba331a1ddf1eea017178cfefaaff9ad72a4b05797fb84bf508b0939aae2972R421-R441);
`routers/web/org/projects.go`
[[3]](diffhunk://#diff-f4279417070a8e33829c338abeb42877500377f490abb1495ae6357d50b6a765R344-R357)
[[4]](diffhunk://#diff-f4279417070a8e33829c338abeb42877500377f490abb1495ae6357d50b6a765R433-R485)
* Updated the project board template to include a milestone filter
dropdown, displaying open and closed milestones and integrating with the
query string for filtering. (`templates/projects/view.tmpl`
[[1]](diffhunk://#diff-e2c7e14d247ce381c352263a8fa639b8341690ff85f6dbebfa166ee3306542feL8-R8)
[[2]](diffhunk://#diff-e2c7e14d247ce381c352263a8fa639b8341690ff85f6dbebfa166ee3306542feR19-R58)

Solves Issue #35224

---------

Signed-off-by: josetduarte <6619440+josetduarte@users.noreply.github.com>
Co-authored-by: joseduarte <joseduarte@aidhound.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

models/issues/milestone_list.go

@@ -24,6 +24,18 @@ func (milestones MilestoneList) getMilestoneIDs() []int64 {
 	return ids
 }
 
+// SplitByOpenClosed splits the milestone list into open and closed milestones
+func (milestones MilestoneList) SplitByOpenClosed() (open, closed MilestoneList) {
+	for _, m := range milestones {
+		if m.IsClosed {
+			closed = append(closed, m)
+		} else {
+			open = append(open, m)
+		}
+	}
+	return open, closed
+}
+
 // FindMilestoneOptions contain options to get milestones
 type FindMilestoneOptions struct {
 	db.ListOptions