Code review comment for lp:~deryck/launchpad/wrong-bug-number-edit-page-439449

This fixes bug 439449. We were using bugtask.id instead of bug.id on
title and heading (label and page_title) for a bug's +edit page.

I added a small test to cover that the correct bug ID is displayed, too.

./bin/test -vv -t xx-bug-edit.txt

= Launchpad lint =

Checking for conflicts. and issues in doctests and templates.
Running jslint, xmllint, pyflakes, and pylint.
Using normal rules.

Linting changed files:
  lib/lp/bugs/browser/bug.py
  lib/lp/bugs/stories/bugs/xx-bug-edit.txt

== Pylint notices ==

lib/lp/bugs/browser/bug.py
    44: [F0401] Unable to import 'lazr.enum' (No module named enum)
    45: [F0401] Unable to import 'lazr.lifecycle.event' (No module named
lifecycle)
    46: [F0401] Unable to import 'lazr.lifecycle.snapshot' (No module
named lifecycle)
    47: [F0401] Unable to import 'lazr.restful.interfaces' (No module
named restful)