4465: add API/Workbench Github links to help modal

4465: add API/Workbench Github links to help modal

Updated the help modal dialog to use the following links for API version
and Workbench version:

https://github.com/curoverse/arvados/tree/#{api_version}/services/api
https://github.com/curoverse/arvados/tree/#{wb_version}/apps/workbench

Refactored some code into a VersionHelper to simplify testing.

Also updated the Rails.configuration.source_version settings for both
API server and Workbench to strip trailing newlines (which were
screwing up the URLs).