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.

Part of single codebase changes.

These are frontend changes.
Use Vue for the import feature UI for "githubish"
providers (GitHub and Gitea).
Add "Go to project" button after a successful import.
Use CI-style status icons and improve spacing of the
table and its component.
Adds ETag polling to the github and gitea import
jobs endpoint.

Bitbucket Server places personal projects in a namespace called
`~username`. This change allows those projects and also strips
them from the GitLab namespace.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53419

Clears the import related columns and code from the Project
model over to the ProjectImportState model

To prevent delays in loading the page and reduce memory usage, limit the
number of projects shown at 25 per page.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/50021

Resolve "Add new project via import by URL auto populates slug but not project name"

Closes #51522

See merge request gitlab-org/gitlab-ce!21770