The unique identifier for the template whose draft should be discarded.

Discard only if the draft is exactly at this revision, taken from a prior
read or mutation. Otherwise ABORTED / STALE_DRAFT_REVISION, with the
current token in the ErrorInfo metadata under current_draft_revision,
which is empty when the draft no longer exists.

Required, unlike on UpdateTemplateDraft, because a discard destroys work
and cannot be undone. The draft may hold someone else's unpublished edits,
so only a draft you have read can be discarded.