Allow importing a management project from Phabricator

Repositories and Project planning features are not tightly coupled in Phabricator. That's why we should allow importing all "tasks" of an instance into a management project with the repository disabled.

For that we should add a new "Importer" button on the imports screen for phabricator.

Similar to the BitBucket server importer we would need the following fields:

• Phabricator URL
• API token

We can get the tasks from maniphest.search. When calling the api, we should specify:

{
  "projects": true,
  "subscribers": true,
  "columns": true
}

We'll need those to add the correct labels in a later step.

For each task imported, we should spin of a separate worker.

In the first step, we will save all "flat fields" of the task on an issue:

• Title
• Description
• Weight
• (simple) State: open/closed, based on the presence of dateClosed. The extended status could become a label.