Add DAG repo to airflow submission response messages

[kevin-bates]

This pull request updates the submission response message to include the GitHub URL that contains the uploaded Airflow DAG. This addition only occurs if the response object contains a git_url attribute.

Note: this logic will need to be revisited once we decouple the platform runtimes from the front-end (as part of #2100), at which time I suspect the response object will also include the message text appropriate to the platform runtime.

How was this pull request tested?

The current automation tests don't cover submission, so this was manually verified using an appropriate pipeline and produced this output:

 $ elyra-pipeline submit --runtime-config af_teacloth1 my_pipeline/kb1_nb_only.pipeline 

────────────────────────────────────────────────────────────────
 Elyra Pipeline Submission
────────────────────────────────────────────────────────────────

Validating pipeline...
 [Warning][Prepare for the apocalypse now!][label] - The node label contains characters that may be replaced by the runtime service. Node labels should start with lower case alphanumeric and contain only lower case alphanumeric, underscores, dots, and dashes. The current property value is 'Prepare for the apocalypse now!'.
 [Warning][Finalize][label] - The node label contains characters that may be replaced by the runtime service. Node labels should start with lower case alphanumeric and contain only lower case alphanumeric, underscores, dots, and dashes. The current property value is 'Finalize'.
 [Warning][Process][label] - The node label contains characters that may be replaced by the runtime service. Node labels should start with lower case alphanumeric and contain only lower case alphanumeric, underscores, dots, and dashes. The current property value is 'Process'.

❯ Job submission succeeded
  Apache Airflow DAG has been pushed to: https://github.com/akchinstc/test-repo/tree/test
  Check the status of your job at: http://teacloth1.fyre.ibm.com:31187
  The results and outputs are in the /kbates/kb1_nb_only-0902134652 
  working directory in http://cloning1.fyre.ibm.com:31467


────────────────────────────────────────────────────────────────
 Elyra Pipeline Submission Complete
────────────────────────────────────────────────────────────────

Resolves #2101

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[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[kiersten-stokes]

LGTM!

[ptitzler]

LGTM