Users aren't notified when their funding goals are reached

[Changaco]

We should send a notification to a user whose income has become greater or equal to their goal. To avoid false-positives, the notification should only be sent if the income will almost certainly continue to be greater or equal than the goal in the coming weeks (i.e. the money has already been received), or if the income has consistently been greater or equal than the goal in the past few weeks.

Related issue: #171.

[cloud-leon]

can i tackle this issue

[Changaco]

@cloud-leon Yes. This one is simple enough.