MSC Multi-Authoring

まだ誰も着手していません。

#### Is your feature request related to a problem? Please describe.
Sometimes, MSCs will gain or have multiple authors. Be it in a corporate/group setting where multiple people have to interact with reviews and threads from the SCT or community. Or in a delegate/"community shepherd" setting where the original author does not have the time or energy to continue working on a MSC.

In these situations, people can still contribute to the MSC, but they crucially do not have the same capabilities as the original PR author, as the following;
- (Un)Resolving threads
- Editing the PR description
- Often required to update implementations, but other reasons may also exist.
- Update the PR branch itself with commits.

#### Describe the solution you'd like.
For the first two issues, I'd like to request that the mscbot is able to respond to commands such as `@mscbot resolve` (on a thread) or `@mscbot update_description ` (as a comment, which then gets hidden after processing).

For the last issue, there is #21.

To activate it, the original author should add `Author: @user_a, @user_b, ...` to the PR description. (Obviously, the `update_description` will not be able to overwrite this part)

まず、mscbot によるプルリクエストコメントとレビュー スレッドの処理を確認し、次にプルリクエストの説明と作成者の権限がどのように表現されているかを追跡します。issue では、スレッドを解決して説明を更新するためのコマンドに加えて、保護されたままにする必要がある Author 宣言が指定されています。認可された共同作成者が要求された操作を実行でき、認可されていないユーザーは実行できなければ完了です。

索引モデルが issue の本文から書いたものです。