[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

templates/swagger/v1_json.tmpl

@@ -2269,6 +2269,47 @@
             "$ref": "#/responses/Branch"
           }
         }
+      },
+      "delete": {
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Delete a specific branch from a repository",
+        "operationId": "repoDeleteBranch",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "branch to delete",
+            "name": "branch",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "204": {
+            "$ref": "#/responses/empty"
+          },
+          "403": {
+            "$ref": "#/responses/error"
+          }
+        }
       }
     },
     "/repos/{owner}/{repo}/collaborators": {