Import alternate bug tracker info from MetaCPAN into RT [rt.cpan.org #80020]

This patch adds the queue Attribute 'DistributionBugtracker' which is
populated using data from the MetaCPAN API.

A two parse approach is taken, one to set or update the information from
MetaCPAN into the RT database and a second that removes the information
from queues that no longer require it.

This work has been sponsored by Shadowcat Systems.

[TRS: Relies on complimentary patches to RT::BugTracker and
RT::BugTracker::Public.  I replaced a call to the 3.8-era LimitAttribute
with the explicit Join and Limits.]

Signed-off-by: Thomas Sibley <trs@bestpractical.com>
Add a note from [rt.cpan.org #80020] so we have a chance of remembering

Otherwise I suspect we'll never remember to check in on the CPAN-API
issue.
More debug messages when updating bugtracker data
Send a User-Agent with MetaCPAN requests

Forces transport to "http" even though it's the default so we guarantee
the ->agent method will work on the transport client if the default
changes in the future.
Add a useful gitignore
Iteratively process bugtracker data instead of loading it all into memory

This lowers memory usage.  The "what attributes need deleting?" question
is pushed the database instead of iterating, in Perl, all queues with
bugtracker attributes.
Merge branch 'alternate-bugtracker-sync' into deploy

Conflicts:
	.gitignore