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>

modules/templates/helper.go

@@ -31,6 +31,7 @@ func NewFuncMap() template.FuncMap {
 		"ctx": func() any { return nil }, // template context function
 
 		"DumpVar": dumpVar,
+		"NIL":     func() any { return nil },
 
 		// -----------------------------------------------------------------
 		// html/template related functions