Make default merge messages configurable

[MarkusAmshove]

Feature Description

Hello there 👋🏻

Currently the default commit messages for merges in pull requests are hard coded within Gitea:

gitea/models/pull.go

Lines 243 to 247 in 20eaca6

	if pr.BaseRepoID == pr.HeadRepoID {
	return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch)
	}
	return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseBranch)

I think it would be great if the default message could be a template, so that we can change them to our needs.

In our specific case the title of a PR has to be in a fixed form, e.g. "Issue #12345: Support commit message templates".
The commits within our repositories also have to comply to that form, so the workflow for merging is always:

1. Press merge button
2. Cut out the first part of the message up to PR title
3. Cut out everything after the PR title

With a template I could just change the template from Merge pull request '{{.Title}}'... to {{.Title}} :-)

Screenshots

No response

[qwerty287]

Duplicate of #11089?