Related issues FE -- Step 7: Add input processing methods to root

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1797

• Add pending related issues input processing methods to RelatedIssuesRoot

Todo

• Add/Update tests
• Support URLs
• Add comment header docs, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1797#note_28856934
• When submitting pendingRelatedIssues, put immediatly in relatedIssues See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1815#note_28924130