Print the dates for each sprint (or week or custom time interval)

In output like

```
Resolved 2 tickets in sprint 1.
Resolved 3 tickets in sprint 2.
Resolved 7 tickets in sprint 3.
Resolved 5 tickets in sprint 4.
Resolved 10 tickets in sprint 5.
```

The numbers in "sprint 1", "sprint 2", etc. aren't meaningful to the user. Let's change it to print the date ranges instead. That would also be useful when you're running the script regularly and saving the results so you can see how accurate the predictions were and whether they're getting better or worse.

For example:

```
Resolved 2 tickets in sprint 2021-03-01 to 2021-03-14.
Resolved 3 tickets in sprint 2021-02-15 to 2021-03-01.
...
```

_Originally posted by @kn-ad in https://github.com/agiledigital-labs/probabilistic-forecast/issues/2#issuecomment-802457658_