Clarify path param naming (#32969)

In history (from some legacy frameworks), both `:name` and `name` are
supported as path path name, `:name` is an alias to `name`.

To make code consistent, now we should only use `name` but not `:name`.

Also added panic check in related functions to make sure the name won't
be abused in case some downstreams still use them.

routers/api/v1/repo/milestone.go

@@ -280,7 +280,7 @@ func DeleteMilestone(ctx *context.APIContext) {
 
 // getMilestoneByIDOrName get milestone by ID and if not available by name
 func getMilestoneByIDOrName(ctx *context.APIContext) *issues_model.Milestone {
-	mile := ctx.PathParam(":id")
+	mile := ctx.PathParam("id")
 	mileID, _ := strconv.ParseInt(mile, 0, 64)
 
 	if mileID != 0 {