User should be able to mute and unmute channels

[holmesworcester]

Right now users can't mute channels. They have to get notifications for all channels.

A user should be able to mute and unmute channels.

When a channel is muted, it should be clear.

We should do this for desktop and mobile. Mobile and desktop mute settings should not affect each other.

Design: https://www.figma.com/file/GmXFtNoihJzIGJMUu5K0WA/Mute-channels-%26-dms?node-id=7%3A2510