MSC Multi-Authoring

#### 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)

Start at mscbot's handling of pull-request comments and review threads, then trace how pull-request descriptions and author permissions are represented. The issue specifies commands for resolving threads and updating descriptions, plus an Author declaration that must remain protected. Done means authorized co-authors can perform the requested actions while unauthorized users cannot.

Written by the indexing model from the issue text.