[API] Add branch delete (#11112)

* use same process as in routers/repo/branch.go/deleteBranch

* make sure default branch can not be deleted

* remove IsDefaultBranch from UI process - it is worth its own pull

* permissions

routers/api/v1/api.go

@@ -664,6 +664,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 				m.Group("/branches", func() {
 					m.Get("", repo.ListBranches)
 					m.Get("/*", context.RepoRefByType(context.RepoRefBranch), repo.GetBranch)
+					m.Delete("/*", reqRepoWriter(models.UnitTypeCode), context.RepoRefByType(context.RepoRefBranch), repo.DeleteBranch)
 				}, reqRepoReader(models.UnitTypeCode))
 				m.Group("/branch_protections", func() {
 					m.Get("", repo.ListBranchProtections)