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).
4465: added api_version_text helper.

Per code review: the source_version returned in the discovery document
may include the string "-modified" if the API server is running from a
locally modified repository.  The api_version_link that we generate for
this version must take that into account.
4465: test for regex link targets

The goal of this story is that the "report issue" dialog includes links
to a Github or Redmine page corresponding to the software versions for
Workbench and the API server, so the test should ensure not just that
there's a link with a given text, but that its target is a Github page
corresponding to a hexadecimal commit hash.
4465: add trailing /? to regexes

Per code review @ #4465-7
Merge branch '4465-workbench-version-links'

Fixes #4465.