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.

app/diagnostics.go

@@ -243,6 +243,7 @@ func trackConfig() {
 		"isdefault_custom_description_text":       isDefault(*utils.Cfg.TeamSettings.CustomDescriptionText, model.TEAM_SETTINGS_DEFAULT_CUSTOM_DESCRIPTION_TEXT),
 		"isdefault_user_status_away_timeout":      isDefault(*utils.Cfg.TeamSettings.UserStatusAwayTimeout, model.TEAM_SETTINGS_DEFAULT_USER_STATUS_AWAY_TIMEOUT),
 		"restrict_private_channel_manage_members": *utils.Cfg.TeamSettings.RestrictPrivateChannelManageMembers,
+		"enable_X_to_leave_channels_from_LHS":     *utils.Cfg.TeamSettings.EnableXToLeaveChannelsFromLHS,
 		"experimental_town_square_is_read_only":   *utils.Cfg.TeamSettings.ExperimentalTownSquareIsReadOnly,
 	})