Setup Phabricator import

What does this MR do?

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 schedul the next page to be imported. But to avoid deadlocks, we only allow 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.

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60562

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created: https://gitlab.com/gitlab-org/gitlab-ce/issues/62516
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

/label security