allow adding inferred issue number to merge request template

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Proposal

Merge request descriptions can now be pre-filled, either

• from commit message/branch name (with issue to be closed and a description from the commit, if available)
• from a default merge request template

The resolution of #353548 (closed) should change the precedence from the former to the latter.

Regardless of the precedence, using a default merge request template now means that the automatically inferred issue number (and description) is lost.

As suggested by @fibbo_ch in #353548 (comment 934180259), this could be solved by making this available as some variable in the templates, e.g. ${CLOSES_ISSUE}.