Booker room_bookings and events default to use termtime timeframe

• add filter to _get_bookings to simplify each operation
• convenience function for getting timeframe (also use it for termtime.get_events)

MISSING: tests for booker.get_room_bookings and booker.get_events operations

Closes: #24 (closed) and #26 (closed)

essmsync/tests/fixtures/booker-bookings-2020-10-05-2021-10-03.json

+[
+  {
+    "booking": {
+      "BatchId": -1,
+      "BookedBy": {
+        "Id": "real-user"
+      },
+      "Description": "Unused Description",
+      "EndTime": "2020-12-03T12:30:00Z",
+      "ExpectedAttendees": 10,
+      "OptimeIndex": 1001,
+      "Room": {
+        "Id": "B001-01-0001"
+      },
+      "StartTime": "2020-12-03T10:15:00Z",
+      "Status": "Approved",
+      "Title": "Booking made by a real person (Michaelmas)",
+      "Type": "Meeting"
+    }
+  },{
+    "booking": {
+      "BatchId": -1,
+      "BookedBy": {
+        "Id": "real-user"
+      },
+      "Description": "Unused Description",
+      "EndTime": "2021-02-01T14:00:00Z",
+      "ExpectedAttendees": 10,
+      "OptimeIndex": 1002,
+      "Room": {
+        "Id": "B001-02-0001"
+      },
+      "StartTime": "2021-02-01T13:00:00Z",
+      "Status": "Approved",
+      "Title": "Booking made by a real person (Lent)",
+      "Type": "Meeting"
+    }
+  },{
+    "booking": {
+      "BatchId": -1,
+      "BookedBy": {
+        "Id": "real-user"
+      },
+      "Description": "Unused Description",
+      "EndTime": "2021-05-20T14:00:00Z",
+      "ExpectedAttendees": 10,
+      "OptimeIndex": 1003,
+      "Room": {
+        "Id": "B002-00-0001"
+      },
+      "StartTime": "2021-05-20T13:00:00Z",
+      "Status": "Approved",
+      "Title": "Booking made by a real person (Easter) - BST",
+      "Type": "Meeting"
+    }
+  },{
+    "booking": {
+      "BatchId": -1,
+      "BookedBy": {
+        "Id": "essm-sync"
+      },
+      "Description": "Unused Description",
+      "EndTime": "2020-12-03T12:30:00Z",
+      "ExpectedAttendees": 10,
+      "OptimeIndex": 1011,
+      "Room": {
+        "Id": "B001-01-0001"
+      },
+      "StartTime": "2020-12-03T10:15:00Z",
+      "Status": "Approved",
+      "Title": "Booking made by a essm-sync (Michaelmas)",
+      "Type": "Meeting"
+    }
+  },{
+    "booking": {
+      "BatchId": -1,
+      "BookedBy": {
+        "Id": "essm-sync"
+      },
+      "Description": "Unused Description",
+      "EndTime": "2021-02-01T14:00:00Z",
+      "ExpectedAttendees": 10,
+      "OptimeIndex": 1012,
+      "Room": {
+        "Id": "B001-02-0001"
+      },
+      "StartTime": "2021-02-01T13:00:00Z",
+      "Status": "Approved",
+      "Title": "Booking made by a essm-sync (Lent)",
+      "Type": "Meeting"
+    }
+  },{
+    "booking": {
+      "BatchId": -1,
+      "BookedBy": {
+        "Id": "essm-sync"
+      },
+      "Description": "Unused Description",
+      "EndTime": "2021-05-20T14:00:00Z",
+      "ExpectedAttendees": 10,
+      "OptimeIndex": 1013,
+      "Room": {
+        "Id": "B002-00-0001"
+      },
+      "StartTime": "2021-05-20T13:00:00Z",
+      "Status": "Approved",
+      "Title": "Booking made by essm-sync (Easter)",
+      "Type": "Meeting"
+    }
+  }
+]