* Add config value EnableXToLeaveChannelsFromLHS that if true displays x to the right of private and public channels on the LHS.
* Displays x to the right of private and public channels on the LHS based on if EnableXToLeaveChannelsFromLHS=true.
* change the tooltip for private and public channels to `Leave channel`
* Add client-side event for when user clicks the "x" button. Different for public, private, DM.
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