Detect if pipeline runs for a GitHub pull request

When using a mirror for CI/CD only we register a pull_request
webhook. When a pull_request webhook is received, if the
source branch SHA matches the actual head of the branch in the
repository we create immediately a new pipeline for the
external pull request. Otherwise we store the
pull request info for when the push webhook is received.

When using "only/except: external_pull_requests" we can detect
if the pipeline has a open pull request on GitHub and create or
not the job based on that.

Use stub_rails_env for Rails.env stubbing

Closes #66966

See merge request gitlab-org/gitlab-ce!32625

Now, when the dns rebinging setting is disabled, we will
allow urls that are not resolvable.

RailsHelpers.stub_rails_env takes care of
stubbing Rails.env

- `edge_nodes` needs to get called on the object
- added `include GlobalID::Identification` in a couple places
- renamed `object` to `item` in spec due to conflict

- Due to https://github.com/exAspArk/batch-loader/pull/32,
we  changed BatchLoader.for into BatchLoader::GraphQL.for
- since our results are wrapped in a BatchLoader::GraphQL,
calling `sync` during authorization is required to get real object
- `graphql` now has it's own authorization system.  Our
`authorized?` method conflicted and required renaming

Improve diff lines count efficiency

Introduce JWTAutheticatable module that can be reused for ai=uthtication
between Pages and GitLab (the same way we use do now for Workhorse).

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/61927.

Creates new event when an epic is created, closed, reopened or
commented.

Add new RailsHelpers module for specs

Closes #66688

See merge request gitlab-org/gitlab-ce!32309