Description

This is a feature that mobile already has, so this would only be a QML and Nim feature.

Add a new button in the context menu when right-clicking a community that is named "Mark community as read". When it is clicked, it marks channels in that community as read and the unread indicators disappear.

Acceptance Criteria

• New button called "Mark community as read" is added to the community context menu
• When clicked, all channels and the community lose their "unread" indicators
• If that community is the only one with notifications, the red dot indicator also disappears