Fix issue with avatar being required once uploaded.

If the user has uploaded an avatar before they can only
change the profile by uploading another avatar that
matches the requirements. This was due to the value in the
validator not being empty if no image was uploaded by
actually being the previously uploaded file.
Add purchase status to purchase listing and hide canceled purchases.

Related to #49.
Merge pull request #46 from EuroPython/fix/broken-avatar-validation

Fix issue with avatar being required once uploaded.
Merge pull request #50 from EuroPython/feature/filtered-purchases

Add purchase status to purchase listing and hide canceled purchases.