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.

model/config.go

@@ -350,6 +350,7 @@ type TeamSettings struct {
 	RestrictPublicChannelDeletion       *string
 	RestrictPrivateChannelDeletion      *string
 	RestrictPrivateChannelManageMembers *string
+	EnableXToLeaveChannelsFromLHS       *bool
 	UserStatusAwayTimeout               *int64
 	MaxChannelsPerTeam                  *int64
 	MaxNotificationsPerChannel          *int64
@@ -810,6 +811,11 @@ func (o *Config) SetDefaults() {
 		*o.TeamSettings.RestrictPrivateChannelManageMembers = PERMISSIONS_ALL
 	}
 
+	if o.TeamSettings.EnableXToLeaveChannelsFromLHS == nil {
+		o.TeamSettings.EnableXToLeaveChannelsFromLHS = new(bool)
+		*o.TeamSettings.EnableXToLeaveChannelsFromLHS = false
+	}
+
 	if o.TeamSettings.UserStatusAwayTimeout == nil {
 		o.TeamSettings.UserStatusAwayTimeout = new(int64)
 		*o.TeamSettings.UserStatusAwayTimeout = TEAM_SETTINGS_DEFAULT_USER_STATUS_AWAY_TIMEOUT