FEAT(client): Add "View Description" context action to channels

[dexgs]

Previously, channel descriptions could only be viewed as a pop-up which does not allow text selection/copying and doesn't allow you to scroll through descriptions to large to fit on your screen.

User comments already support this use case via the "View Comment" context menu action on users which opens their comment in a separate window.

This commit adds a "View Description" context menu action to channels which works the same way as the "View Comment" action.

Fixes #6525

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