Refactor `District.number` to an `IntegerField`

[lalver1]

This is so that the field type is consistent with the URL argument type that is passed into the districts:district URL. It also makes for cleaner queries, such as District.objects.get(number__iexact=self.kwargs["district"]) in the DistrictView class, since both types will match.

[lalver1]

Reviewed this one more time and it seems it's not relevant anymore.

The reason is that in #102, a change was made to use re_path to capture the district_number part of the URL. The captured value is passed as a string to DistrictView and consequently self.kwargs["district_number"] is a string. Therefore, changing District.number to an IntegerField is not needed anymore.

Also, originally we thought that we would implement validation of a district number by checking if it is within the range 1 to 12, so an integer type would have been useful. But we implemented validation using a regular expression instead. This aligns with our original thought that district_number primarily serves as an identifier and is better modeled as a TextField.