Use relative URL for Markdown references, except in mails.

app/helpers/issues_helper.rb

app/models/project_services/gitlab_issue_tracker_service.rb

app/models/project_services/issue_tracker_service.rb

app/views/layouts/nav/_project.html.haml

app/views/notify/_note_message.html.haml

app/views/notify/new_issue_email.html.haml

app/views/notify/new_merge_request_email.html.haml

app/views/projects/_dropdown.html.haml

lib/gitlab/markdown.rb

spec/helpers/gitlab_markdown_helper_spec.rb

...els/project_services/gitlab_issue_tracker_service_spec.rb