dspace_statistics_api: Add support for date ranges to /items

You can now POST a JSON request to /items with a list of items and
a date range. This allows the possibility to get view and download
statistics for arbitrary items and arbitrary date ranges.

The JSON request should be in the following format:

    {
        "limit": 100,
        "page": 0,
        "dateFrom": "2020-01-01T00:00:00Z",
        "dateTo": "2020-09-09T00:00:00Z",
        "items": [
            "f44cf173-2344-4eb2-8f00-ee55df32c76f",
            "2324aa41-e9de-4a2b-bc36-16241464683e",
            "8542f9da-9ce1-4614-abf4-f2e3fdb4b305",
            "0fe573e7-042a-4240-a4d9-753b61233908"
        ]
    }

The limit, page, and date parameters are all optional. By default
it will use a limit of 100, page 0, and [* TO *] Solr date range.

dspace_statistics_api/util.py

@@ -51,3 +51,20 @@ def get_statistics_shards():
     # seem to mind if the shards query parameter is empty and I haven't seen
     # any negative performance impact so this should be fine.
     return shards
+
+
+def is_valid_date(date):
+    import datetime
+    import falcon
+
+    try:
+        # Solr date format is: 2020-01-01T00:00:00Z
+        # See: https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior
+        datetime.datetime.strptime(date, "%Y-%m-%dT%H:%M:%SZ")
+
+        return True
+    except ValueError:
+        raise falcon.HTTPBadRequest(
+            title="Invalid parameter",
+            description=f"Invalid date format: {date}. The value must be in format: 2020-01-01T00:00:00Z.",
+        )