PR templates - support for inputs

Hi,

This is a request for a feature:
I'd like to have a possibility to specify a fields, that needs to be/or can be entered easily in a PR template.

For example, I want a developer to link a Jira issue in a PR it is related to (we have it in commit, ideally in each one, but it easier to have it in PR description).

So similarly to simple checkbox, I'd like to have easy way in "Preview" tab for PR authors to enter required inputs.

```markdown
Jira issue: {TODO}
```

Maybe a first version might be as simple as this: the text between `{}` will be replaced (so it will work first time only) on first edit, Maybe someone could extend this for edits.

Also I'd like to have an option to mark it mandatory, for example

```markdown
Jira issue: {*TODO}
```

might be a version for mandatory input, so only draft PR can be created if not filled in.

Thanks!

Commencez par localiser le point d’entrée de la gestion de PR-template dans ce JavaScript GitHub wrapper et déterminez si le comportement de saisie du template est exposé ici ou relève de GitHub lui-même. Définissez comment doivent fonctionner le remplacement des espaces réservés, les champs obligatoires, la première modification et les modifications ultérieures ; l’achèvement nécessiterait une conception approuvée et des tests pour ces comportements.

Rédigé par le modèle d'indexation à partir du texte de l'issue.