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.

These are backend 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.

Users may specify an OAuth2 callback with a custom name, such as
AWSCognito, but Rails will reject this with the following message:

```
'import/AWSCognito' is not a supported controller name. This can
lead to potential routing problems. See
http://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use
```

To avoid these errors, we can just downcase all the provider names.
Note that this will make it impossible to specify a duplicate name with
different cases.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57156

To prevent an OAuth2 covert redirect vulnerability, this commit adds and
uses an alias for the GitHub and BitBucket OAuth2 callback URLs to the
following paths:

GitHub: /users/auth/-/import/github
Bitbucket: /users/auth/-/import/bitbucket

This allows admins to put a more restrictive callback URL in the OAuth2
configuration settings. Instead of https://example.com, admins can now use:

https://example.com/users/auth

It's possible but not trivial to change Devise and OmniAuth to use a
different prefix for callback URLs instead of /users/auth. For now,
aliasing the import URLs under the /users/auth namespace should suffice.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56663

To prevent an OAuth2 covert redirect vulnerability, this commit adds and
uses an alias for the GitHub and BitBucket OAuth2 callback URLs to the
following paths:

GitHub: /users/auth/-/import/github
Bitbucket: /users/auth/-/import/bitbucket

This allows admins to put a more restrictive callback URL in the OAuth2
configuration settings. Instead of https://example.com, admins can now use:

https://example.com/users/auth

It's possible but not trivial to change Devise and OmniAuth to use a
different prefix for callback URLs instead of /users/auth. For now,
aliasing the import URLs under the /users/auth namespace should suffice.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56663

It allows user to automatically import multiple repositories
with nested structure by uploading a manifest xml file.

AOSP project was used as an example during development of this feature.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>