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.

Avoid calling freeze on already frozen strings in lib/gitlab

See merge request gitlab-org/gitlab-ce!32637

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

- `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

When using /due quick action with an invalid date a meaninful error
message is shown.

- 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

The current state of group issue boards does not show the "Add issues"
button on the UI for users that are reporters of group child projects.

(cherry picked from commit 0bf7fecdee5a200eeb4f27b4888a57ceee63bde9)