Hide variables that should not be overridden

Issue information

From user: hestenet

Related to !197 (merged)

Contribution record

Fork management

Problem/Motivation

We have two sets of GitLab CI variables in our template configuration:
Variables prefixed with _ sort to the top of the list, and are meant to be overridden by project maintainers for adhoc tests and general configuration.
Variables without the prefix are meant to be updated centrally by the DA in these templates, and should not be overriden.

However, the UI allows maintainers access to both variables.

Proposed resolution

File an upstream request with GitLab to designate certain variables as 'not overridable'

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