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!

首先定位此 JavaScript GitHub wrapper 中处理 PR-template 的入口点，并确定模板输入行为是在此处提供，还是属于 GitHub 本身。定义占位符替换、必填字段、首次编辑和后续编辑应如何工作；完成这项工作需要就设计达成一致，并为这些行为编写测试。

由索引模型根据 Issue 内容生成。