Move calculation of gitlab templates version out of *show-ci-variables

Problem/Motivation

The reusable code snippet .show-ci-variables does more than just "show" the variables, it executes a curl on scripts/extract-version.php then runs that script. If this snippet is used in another job, eg custom after_script, with the intention of displaying the Templates version, then it is not only wasting resources to get the script and execute it, but it can cause errors and/or not run correctly, and give confusing infomation in the log.

Proposed resolution

It would be preferable to move the curl and execution of the script into a different code reference, say something like .extract-version, and leave the .show-ci-variables to just do the printing. Then the latter can be called in other places too, without the problem of doing the curl and executing the script.

Remaining tasks

User interface changes

API changes

Data model changes