[Refactor] move APIFormat() of Issue and Label to convert package (#10423)

* Label: delete .APIFormat() and use convert.ToLabel()

* move issue APIFormat too

* add missing one

* move TEST too

* handle error -> return empty APIIssue

* Apply suggestions from code review

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>

models/issue_label.go

@@ -12,8 +12,6 @@ import (
 	"strconv"
 	"strings"
 
-	api "code.gitea.io/gitea/modules/structs"
-
 	"xorm.io/builder"
 	"xorm.io/xorm"
 )
@@ -37,16 +35,6 @@ type Label struct {
 	IsExcluded      bool   `xorm:"-"`
 }
 
-// APIFormat converts a Label to the api.Label format
-func (label *Label) APIFormat() *api.Label {
-	return &api.Label{
-		ID:          label.ID,
-		Name:        label.Name,
-		Color:       strings.TrimLeft(label.Color, "#"),
-		Description: label.Description,
-	}
-}
-
 // GetLabelTemplateFile loads the label template file by given name,
 // then parses and returns a list of name-color pairs and optionally description.
 func GetLabelTemplateFile(name string) ([][3]string, error) {