Make avatar_url unguessable

[vtran99]

Request to modify avatar_url to make it unguessable as with the current '/user//picture-default'
it's too easy for anyone to access other user's profile picture by changing randomly number.

Notes:
This change breaks the tests on users as dataset used contain static value of 'avatar_url' which can't be set with the change.
What I can do is:

• either remove avatar_url from the tests;
• either set a fake value inside the test to match the value in the dataset;
• other suggestion ?

[ThiefMaster]

I did a few cleanups and also added mocking the serializer so the URLs do not change between test runs.

[ThiefMaster]

@vtran99 When backporting this to your fork, make sure to also apply this commit fixing a bug I only discovered after merging the PR: da634d3