MSC Multi-Authoring

Dieses Issue hat noch niemand übernommen.

#### 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)

Beginne mit der Verarbeitung von Pull-Request-Kommentaren und Review-Threads durch mscbot und verfolge anschließend, wie Pull-Request-Beschreibungen und Autorenberechtigungen dargestellt werden. Das Issue gibt Befehle zum Auflösen von Threads und Aktualisieren von Beschreibungen vor sowie eine Author-Deklaration, die geschützt bleiben muss. Als erledigt gilt die Aufgabe, wenn autorisierte Co-Autoren die angeforderten Aktionen ausführen können, nicht autorisierte Benutzer jedoch nicht.

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