Add `http` method to `BaseHandler`

The `BaseHandler.http` method behaves similar to pecan's abort function,
and allows end users to raise http response with custom status code,
custom message and custom headers.

Related to blueprint reduce-dependence-on-webpy
Closes-Bug: #1277174

Change-Id: I8759ada1451446602c107b26a4d7e7fabd99c568
Merge "Add `http` method to `BaseHandler`"