Refactor sidebar assignee&milestone&project selectors (#32465)

Follow #32460

Now the code could be much clearer than before and easier to maintain. A
lot of legacy code is removed.

Manually tested.

This PR is large enough, that fine tunes could be deferred to the future if
there is no bug found or design problem.

Screenshots:

<details>

![image](https://github.com/user-attachments/assets/35f4ab7b-1bc0-4bad-a73c-a4569328303c)

</details>

templates/repo/issue/milestone/select_menu.tmpl

@@ -1,38 +0,0 @@
-{{if or .OpenMilestones .ClosedMilestones}}
-	<div class="ui icon search input">
-		<i class="icon">{{svg "octicon-search" 16}}</i>
-		<input type="text" placeholder="{{ctx.Locale.Tr "repo.issues.filter_milestones"}}">
-	</div>
-	<div class="divider"></div>
-{{end}}
-<div class="no-select item">{{ctx.Locale.Tr "repo.issues.new.clear_milestone"}}</div>
-{{if and (not .OpenMilestones) (not .ClosedMilestones)}}
-	<div class="disabled item">
-		{{ctx.Locale.Tr "repo.issues.new.no_items"}}
-	</div>
-{{else}}
-	{{if .OpenMilestones}}
-		<div class="divider"></div>
-		<div class="header">
-			{{ctx.Locale.Tr "repo.issues.new.open_milestone"}}
-		</div>
-		{{range .OpenMilestones}}
-			<a class="item" data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}">
-				{{svg "octicon-milestone" 16 "tw-mr-1"}}
-				{{.Name}}
-			</a>
-		{{end}}
-	{{end}}
-	{{if .ClosedMilestones}}
-		<div class="divider"></div>
-		<div class="header">
-			{{ctx.Locale.Tr "repo.issues.new.closed_milestone"}}
-		</div>
-		{{range .ClosedMilestones}}
-			<a class="item" data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}">
-				{{svg "octicon-milestone" 16 "tw-mr-1"}}
-				{{.Name}}
-			</a>
-		{{end}}
-	{{end}}
-{{end}}