Fix parsing of quoted tags with spaces

Fixes gh-4
Implement tag mapping

Support a simple mapping to clean up and harmonize tags while migrating.
Clean up formatting and redundancy in log messages
Log error details on UnprocessableEntity error
Don't bork if assignee is not a collaborator

... instead just add a mention of the assignee to the body.

For general robustness to discrepancies between user mapping and current
collaborators, but also for the specific use case of test-migrating into
a fork with collaborators removed (to spare them the notification flood)
Log error details also when creating a milestone
Fetch all github milestones even if >30
Cache milestones fetched, not just those created

Matters when resuming a migration.
Merge pull request #5 from gthb/patch-1

Several fixes and minor improvements
Error logging on bad create_gh_issue return value
Handle milestone_id == 0 the same as nil

A `milestone_id` of `0` (instead of `nil`) has been seen coming out of
the Lighthouse API, for a ticket that has no milestone in the Lighthouse
web interface. A request to `.../milestones/0.json` then yields a 404,
causing the migration to error out.

So, interpret `milestone_id == 0` the same as `milestone_id.nil?`
Sleep and retry once on GitHub API 502 Bad Gateway

... in the three places where I encountered it.
Log args when create_gh_issue returns bogus value

To aid troubleshooting.