Add ability to reorder issues

See merge request gitlab-org/gitlab-ce!29012

to support manual sorting on the frontend

Updating multiple application settings panels through
a single action causes the incorrect action to be shown
when there are errors.  Instead, make each panel action
handle both updating and display.

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

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

Move some project routes under - scope

See merge request gitlab-org/gitlab-ce!28830

- Select notification email by walking up group/subgroup path
- Add settings UI to set group email notification address
- Add tests

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.