This sets up all the basics for importing Phabricator tasks into
GitLab issues.
To import all tasks from a Phabricator instance into GitLab, we'll
import all of them into a new project that will have its repository
disabled.
The import is hooked into a regular ProjectImport setup, but similar
to the GitHub parallel importer takes care of all the imports itself.
In this iteration, we're importing each page of tasks in a separate
sidekiq job.
The first thing we do when requesting a new page of tasks is schedule
the next page to be imported. But to avoid deadlocks, we only allow a
single job per worker type to run at the same time.
For now we're only importing basic Issue information, this should be
extended to richer information.