Simplify PR titles, maybe making them configurable

[ssbarnea]

In its current form the PR produced for a backport looked like:

[PR #3230/5e717ce9 backport][stable/3.4] Ensure SystemExit is captured on driver initialization #3253

That is producing long/hard to read commit messages or release notes (release-drafter) and I would like to make it possible to simplify them.

As I am aware that it would close to impossible to find a one-size-pleases-all solution, maybe having a templated configurable field for title would sort the issue.

Including original backport link in body is essential but I am inclined to believe that something like backport: {src_title} or {src_title} backport #123 would be enough as they would produce easy to read lists.

[webknjaz]

I think I've heard this feature request before or maybe I've thought about it myself. The main concern is that accepting user-defined templates is potentially harmful. So this either needs a few restricted substitutions or maybe to just expose parts of the string as separate config settings.
Anyway, I'd need to think about this very carefully or at least read somebody else's write-up on the possible problems and solutions.