Switch to numeric bug IDs on Launchpad

Bug Repo Syncer should switch to numeric bug IDs on Launchpad.

 * No additional field `bug_link_id` necessary in the bug representations
 * Referring to bugs from the command line would become much more comfortable.
   One could use the same, relatively short numeric bug ID, as in the web
   interface.
 * Switching between "staging" and "qastaging" would become very easy. They use
   different self links, but the same numeric bug IDs.
 * The algorithm for loading bugs would become only slightly more complicated
   and slightly slower.

Find bug by numeric bug ID:
{{{
    bug_id = 934181

    lp_bug = launchpad.bugs[bug_id]
    for lp_task in lp_bug.bug_tasks:
        if lp_task.target == self.project:
            break
    #do things with ``lp_bug`` and ``lp_task``
}}}

Getting the numeric bug ID from a bug:
{{{
    bug_id = lp_bug.id
}}}

== Related ==
[@[@BUGLINK "Bug #274"@lp @]@]
bug #934724