Change dynamic captcha registration rates calculation

[lmuntaner]

Motivation

The dynamic captcha is triggered without user registration spikes.

This is due to the current calculation of the current rate which doesn't use the current interval, and instead uses as interval the difference between now and the oldest point.

For example:

• current_rate_interval is 5 minutes.
• There are no registrations in the last 5 minutes.
• Then two new registrations are added in 2 seconds.
• The calculation right afterwards will be 2 seconds. Therefore, the rate would be 2 registrations / 2 seconds -> 1. Instead of 2 registrations / 5 minutes.

Changes

• Change the rate calculation in registration_rates.

Tests

• Change the test expectations.

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🟡 Some screens were changed

[lmuntaner]

@sea-snake please review

[sea-snake]

As discussed, this change will affect the behavior when window size configuration has changed since this change doesn't take into account how to handle missing (pruned) data points.

Overall, as long as we don't change the window size configuration till we have an implementation handling the missing data points, we shouldn't run into issues.