dspace_statistics_api/app.py: Refactor for testability

I thought it was clever to only import these in the on_post handler
because they aren't needed elsewhere, but it turns out that this is
not a common pattern and even causes problems with testability.

First, if the imports are at the top of the file as PEP8 recommends,
then the WSGI server will import them once when it loads the app and
they remain in memory for the lifecycle of the app. If the imports
are in the on_post handler they would be re-imported on every request!

Second, this pattern of importing in a method makes it tricky to use
object patching in mocks.

See: https://www.python.org/dev/peps/pep-0008/#imports

dspace_statistics_api/app.py

@@ -1,6 +1,8 @@
 import falcon
 
 from .database import DatabaseManager
+from .items import get_views
+from .items import get_downloads
 from .util import validate_items_post_parameters
 
 
@@ -60,9 +62,6 @@ class AllItemsResource:
     def on_post(self, req, resp):
         """Handles POST requests"""
 
-        from .items import get_views
-        from .items import get_downloads
-
         # Build the Solr date string, ie: [* TO *]
         if req.context.dateFrom and req.context.dateTo:
             solr_date_string = f"[{req.context.dateFrom} TO {req.context.dateTo}]"