Use project.IconName instead of repeated unreadable if-else chains (#23538)

The project type will be changed in
https://github.com/go-gitea/gitea/pull/23353, so the old fix
https://github.com/go-gitea/gitea/pull/23325 will not work as well.

And I also found that there were some problems in the old fix....

---------

Co-authored-by: Lauris BH <lauris@nix.lv>

models/project/project.go

@@ -144,10 +144,21 @@ func (p *Project) Link() string {
 	return ""
 }
 
+func (p *Project) IconName() string {
+	if p.IsRepositoryProject() {
+		return "octicon-project"
+	}
+	return "octicon-project-symlink"
+}
+
 func (p *Project) IsOrganizationProject() bool {
 	return p.Type == TypeOrganization
 }
 
+func (p *Project) IsRepositoryProject() bool {
+	return p.Type == TypeRepository
+}
+
 func init() {
 	db.RegisterModel(new(Project))
 }