User should be able to create channels for specific roles

[holmesworcester]

Many organizations have channels that are only accessible to a specific role. Discord lets you make channels that are only accessible to a role, and set this when you create a private channel.

First you can choose to make a new channel private:

Then you can choose who has access:

Related: #1704

[jgaylor]

Here's the latest Loom & Figma

I still have lower-level interactions and details to define, but can come back to it after you get feedback.

[jgaylor]

Here's the latest Figma & Loom (this loom only covers updates and channel info)

[jgaylor]

Here's the latest Figma and Loom

[ethanc8]

On "remove role", it would be good to show which members would retain their permissions, and maybe leave buttons to remove those members.