Avoid save base64 images on MongoDb

[RodriSanchez1]

When a user uploads an image while offline, it is stored on the device as a base64 image. When the board is stored or updated in the database, the image is also saved as base64. This issue increases the database size and fills up the user's device memory.

Fix:
We must implement a functionality to store the image as a blob in the Cboard blob storage before upserting the board into the database.