MM-27184 deprecate model.SetExpireInDays (#15165)

Mobile users were having their sessions unexpectedly expired, despite having ServiceSettings.ExtendSessionLengthWithActivity enabled. 

Every time a mobile app is opened it called `/api/v4/sessions/device` which calls attachDeviceId which calls `(*Session)SetExpireInDays`. This code above assumed the expiry should be relative to CreateAt which is incorrect when ExtendSessionLengthWithActivity is enabled. Therefore, every time the mobile app was opened, the maximum expiry was set in memory to CreateAt + session_length, even if the session was extended.

(*Session)SetExpireInDays is now deprecated and replaced with (*App)SetSessionExpireInDays which takes into account the ExtendSessionLengthWithActivity setting.

app/app_iface.go

@@ -275,6 +275,10 @@ type AppIface interface {
 	SetBotIconImage(botUserId string, file io.ReadSeeker) *model.AppError
 	// SetBotIconImageFromMultiPartFile sets LHS icon for a bot.
 	SetBotIconImageFromMultiPartFile(botUserId string, imageData *multipart.FileHeader) *model.AppError
+	// SetSessionExpireInDays sets the session's expiry the specified number of days
+	// relative to either the session creation date or the current time, depending
+	// on the `ExtendSessionOnActivity` config setting.
+	SetSessionExpireInDays(session *model.Session, days int)
 	// SetStatusLastActivityAt sets the last activity at for a user on the local app server and updates
 	// status to away if needed. Used by the WS to set status to away if an 'online' device disconnects
 	// while an 'away' device is still connected