Statistics: Rename "Users" to "Registrations

### Current Behavior
As per Ian's clarification in his comment below we should add a "New during" prefix for the Statistics metric row that relates to the selected time period to clarify that the second metric row relates to new events (New user == Completed registration) during that timeframe.

Original ticket title was _Statistics: Rename "Users" to "Registrations"_
Original ticket description was:
The first graph on statistics shows "Users" which probably to the majority of admin users is expected to show the users visiting the community at the given day:

In fact this metric shows the number of registrations on that day.

### Steps to Reproduce

Take a look at your statistics graph in the admin backend

### Expected Behavior

The current metric shown should be renamed "Registrations" for clarity. Mockup with the changed copy:

(A separate "(active) users" graph would also be very helpful, but is out of scope for this report)

### Environment

- Flarum version: 1.8.19 & 2.0.0-rc.5

Start in the admin backend statistics graph described in the issue and trace the label for the registrations metric and its selected time-period row. Confirm the current wording against the screenshots, then update the labels so the metric is identified as registrations and the period-specific row has the requested “New during” prefix; verify the graph displays the clarified copy.

Written by the indexing model from the issue text.