Need a way to determine what release of checkout_externals was used in a specific CESM tag

### Summary of Issue:
External components which maintain standalone capability need to maintain the checkout_externals version. Typically for CAM's case we take a previously tested CESM tag and duplicate the externals used in it. This is now being extended to needing to also maintain the version of checkout_externals. We want to identify and use the version of checkout_externals that was used in a specific CESM tag. Being able to check this easily, becomes more important as time goes on and checkout_externals is only updated sporadically - we need an easy way to determine that it has been updated.
### Expected behavior and actual behavior:
An ideal solution would be for the plans page to have a location somewhere on each tag's page which indicates the checkout_externals which was used. Perhaps this could be populated (automatically?) when checkout_externals is brought into the CESM tag. Currently, there is no good way to determine the checkout_externals tag other than cloning cesm and mining the log.
### Steps to reproduce the problem. Include externals description file(s) and link to public repository):
N/A
### What is the changeset ID of the code, and the machine you are using:
N/A
### have you modified the code? If so, it must be committed and available for testing:
N/A
### Screen output or log file showing the error message and context:
N/A

@bjandre suggested that I bring @mvertens , @fischer-ncar , @bertinia and possibly @jedwards4b into this discussion.