Allow including `Reviewed-on`/`Reviewed-by` lines for custom merge messages (#31211)

This PR introduces the `ReviewedOn` and `ReviewedBy` variables for the
default merge message templates (e.g.,
`.gitea/default_merge_message/MERGE_TEMPLATE.md`).

This allows customizing the default merge messages while retaining these
trailers.

This also moves the associated logic out of `pull.tmpl` into the
relevant Go function.

This is a first contribution towards #11077.

---

For illustration, this allows to recreate the "default default" merge
message with the following template:
```
.gitea/default_merge_message/MERGE_TEMPLATE.md
Merge pull request '${PullRequestTitle}' (${PullRequestReference}) from ${HeadBranch} into ${BaseBranch}

${ReviewedOn}
${ReviewedBy}
```

1 files changed, 2 insertions, 0 deletions

diff --git a/docs/content/usage/merge-message-templates.en-us.md b/docs/content/usage/merge-message-templates.en-us.md
index fbdbd136f8..5116be3387 100644
--- a/docs/content/usage/merge-message-templates.en-us.md
+++ b/docs/content/usage/merge-message-templates.en-us.md
@@ -44,6 +44,8 @@ You can use the following variables enclosed in `${}` inside these templates whi
 - PullRequestIndex: Pull request's index number
 - PullRequestReference: Pull request's reference char with index number. i.e. #1, !2
 - ClosingIssues: return a string contains all issues which will be closed by this pull request i.e. `close #1, close #2`
+- ReviewedOn: Which pull request this commit belongs to. For example `Reviewed-on: https://gitea.com/foo/bar/pulls/1`
+- ReviewedBy: Who approved the pull request before the merge. For example `Reviewed-by: Jane Doe <jane.doe@example.com>`
 
 ## Rebase