Add x to be able to leave public and private channels (#7145)

* 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.

config/default.json

@@ -71,6 +71,7 @@
         "RestrictPublicChannelDeletion": "all",
         "RestrictPrivateChannelDeletion": "all",
         "RestrictPrivateChannelManageMembers": "all",
+        "EnableXToLeaveChannelsFromLHS": false,
         "UserStatusAwayTimeout": 300,
         "MaxChannelsPerTeam": 2000,
         "MaxNotificationsPerChannel": 1000,