POST /v1/user/subscriptions overwrites current subscriptions

[Alessandro100]

When subscribing to a feed, it should not overwrite the previous feeds

How to reproduce

• call POST /v1/user/subscriptions for a feed A
• see your subscriptions GET /v1/user/subscriptions should see feed A
• call POST /v1/user/subscriptions for a feed B
• see your subscriptions `GET /v1/user/subscriptions

Expected results

• You see subscriptions for feed A and feed B

Actual result

• You only see subscriptions for feed B