[MM-17422] Added code to update, delete last bot_icon time in model.bot. (#12229)

Display LHS bot Icon in web app. As part of mentioned task, Added LastIconUpdate variable in model.bot to store last update time of icon. Also added code to update/delete value of the mentioned variable when setting/deleting bot icon.

model/user.go

@@ -87,6 +87,7 @@ type User struct {
 	LastActivityAt         int64     `db:"-" json:"last_activity_at,omitempty"`
 	IsBot                  bool      `db:"-" json:"is_bot,omitempty"`
 	BotDescription         string    `db:"-" json:"bot_description,omitempty"`
+	BotLastIconUpdate      int64     `db:"-" json:"bot_last_icon_update,omitempty"`
 	TermsOfServiceId       string    `db:"-" json:"terms_of_service_id,omitempty"`
 	TermsOfServiceCreateAt int64     `db:"-" json:"terms_of_service_create_at,omitempty"`
 }
@@ -487,7 +488,7 @@ func (u *UserAuth) ToJson() string {
 
 // Generate a valid strong etag so the browser can cache the results
 func (u *User) Etag(showFullName, showEmail bool) string {
-	return Etag(u.Id, u.UpdateAt, u.TermsOfServiceId, u.TermsOfServiceCreateAt, showFullName, showEmail)
+	return Etag(u.Id, u.UpdateAt, u.TermsOfServiceId, u.TermsOfServiceCreateAt, showFullName, showEmail, u.BotLastIconUpdate)
 }
 
 // Remove any private data from the user object