[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_assignees.go

@@ -19,6 +19,11 @@ type IssueAssignees struct {
 	IssueID    int64 `xorm:"INDEX"`
 }
 
+// LoadAssignees load assignees of this issue.
+func (issue *Issue) LoadAssignees() error {
+	return issue.loadAssignees(x)
+}
+
 // This loads all assignees of an issue
 func (issue *Issue) loadAssignees(e Engine) (err error) {
 	// Reset maybe preexisting assignees