[API] generalize list header (#16551)

* Add info about list endpoints to CONTRIBUTING.md

* Let all list endpoints return X-Total-Count header 

* Add TODOs for GetCombinedCommitStatusByRef

* Fix models/issue_stopwatch.go

* Rrefactor models.ListDeployKeys

* Introduce helper func and use them for SetLinkHeader related func

routers/api/v1/user/user.go

@@ -6,7 +6,6 @@
 package user
 
 import (
-	"fmt"
 	"net/http"
 
 	"code.gitea.io/gitea/models"
@@ -73,8 +72,7 @@ func Search(ctx *context.APIContext) {
 	}
 
 	ctx.SetLinkHeader(int(maxResults), listOptions.PageSize)
-	ctx.Header().Set("X-Total-Count", fmt.Sprintf("%d", maxResults))
-	ctx.Header().Set("Access-Control-Expose-Headers", "X-Total-Count, Link")
+	ctx.SetTotalCountHeader(maxResults)
 
 	ctx.JSON(http.StatusOK, map[string]interface{}{
 		"ok":   true,